A. Legitimacy of Marriage
Evidence for the legitimacy of nikāḥ (marriage) comes from the Qur'ān and the Sunnah. In fact, the Qur'ān is replete with verses which command it, make the believers desirous of it and point to mercy, wisdom and great bounties of Allāh the Almighty who has enjoined it on the believers and created them with a natural tendency to engage in it. Almighty Allāh says,
"And marry those among you who are single and the righteous ones among your male slaves and female slaves."
In this verse, Allāh commands the believers to marry men and women who are not married  as well as the righteous people among the male and female slaves who are under their guardianship. Commenting on this verse, As-Sa'dī writes, "Almighty Allāh commands the guardians as well as the masters to marry off the 'ayāmā', that is those who are not married, male and female, widows and virgins; therefore, the orphans' relatives and guardians ought to marry off whomever needs to get married from among those they are required to support. That they are commanded to marry off those who are under their guardianship, they are a fortiori commanded to get married themselves." 
Allāh also says, "And if you fear that you will not be able to deal justly with the orphan girls, then marry women of your choice, two or three or four; but if you fear that you will not be able to deal justly [with them], then [marry only] one or those your right hands possess (i.e. slaves)." 
"And do not prevent them from remarrying their [former] husbands…"
Almighty Allāh reminds the believers of the numerous blessings and favours He has bestowed upon them; one such favours was that He has made for them mates and companions of their own nature, and made for them, out of them, sons and daughters and grandchildren. Allāh says,
"O mankind, fear your Lord, who created you from a single person, created, of like nature, his mate, and from them twain scattered [like seeds] countless men and women…"
"And Allāh has made for you from yourselves mates and has made for you from your mates sons and daughters and grandchildren and has provided for you sustenance of the best; will they then believe in falsehood and be ungrateful for Allāh’s favours?"
"Glory to Allāh, who created in pairs all things that the earth produces, as well as their own [human] kind and [other] things of which they have no knowledge." 
Almighty Allāh has also made marriage one of the practices of His messengers whom He commends thus: "And indeed We sent messengers before you and We gave them wives and children."
He also commends His believing servants who ask Him to grant them children, thus: "And those who pray, 'Our Lord! Grant unto us wives and offspring who will be the comfort of our eyes, and give us [the grace] to lead the righteous.'" 
He also commends them thus: "Successful indeed are the believers…who abstain from sex, except with those joined to them in the marriage bond, or those whom their right hands possess, for [in their case] they are indeed free from blame…" The Sunnah is also replete with traditions to this effect. The following are but a few examples.
Abdullāh b. Mas'ūd (raḍiyallāhu anhu) narrated that the Prophet (ṣallallāhu `alayhi wa-sallam) said, "O Young men, those of you who have the ability to marry  should marry, for [matrimony] helps in lowering the gaze and protecting the private parts; but whoever among you cannot afford [to marry] should fast, for indeed it is a shield  for him." 
The Prophet (ṣallallāhu `alayhi wa-sallam) also said, "Marry women who are loving and fertile, for I will be proud of your great numbers before the nations." 
Regarding the unanimous agreement of Muslim scholars, Ibn Qudāmah said, "Muslims are unanimously agreed that marriage (nikāḥ) is a legitimate institution, but our companions are not in agreement as to its obligation. The well-known opinion of [our] school of jurisprudence] is that it is not obligatory unless one fears one may engage in unlawful acts if one does not get married, in which case one must preserve one's chastity [by getting married]; this is also the opinion of the majority of Muslim scholars." 
B. Prohibition of Zinā and its Detrimental Effects Definition of Zinā
We shall consider here the linguistic and technical definitions of zinā.
a. Linguistic Definition of Zinā
The Arabic word 'zinā' has three meanings :
In its first sense, it means 'narrowness, tightness, closeness'. The word 'zannā'' is used to refer to someone who suffers from urinary retention. 
In its second sense, it means adultery or fornication, namely having intercourse with a woman without a valid contract and doubt [as to the woman's identity]. 
In its third sense, it refers to any act other than sexual intercourse with a woman who is considered ajnabiyyah . The Prophet (ṣallallāhu `alayhi wa-sallam) said, "Allāh has written for the son of Adam his inevitable share of adultery whether he is aware of it or not: The adultery of the eye is the looking [at something which is sinful to look at], and the adultery of the tongue is to utter [what is unlawful to utter], and the inner self wishes and longs for [adultery] and the private parts turn that into reality or refrain from submitting to the temptation." 
b. Technical Definition of Zinā
The technical definition of zinā is more or less the same as its linguistic definition, namely having intercourse with a woman without a valid contract and doubt [as to the woman's identity], except for the legal restrictions imposed on the doer or the deed itself. Perhaps the best definition ever furnished in this regard is that given by Al-Jurjānī (raḥimahullāh) who said, "Zinā can be defined as having intercourse in the part (vagina) without a right and a doubt."  By 'intercourse', we exclude any act that does not amount to it, such as kissing, fondling and embracing.
By 'in the front part', we exclude anal intercourse, which is a form of a homosexual act.
By 'without a right' is meant the right to intercourse whereby the husband or master has the right to intercourse with the wife or those whom his right hand possesses.
By 'a doubt' is meant mistaken marriage or possession, such as when one believes it is absolutely lawful to have intercourse in cases when he is not fully aware of what has happened; an example of this is when a woman is given in marriage to someone and he is told that it is his wife, and so he has intercourse with her on the thought that she is his wife. This, in fact, is not considered an act of zinā and thus he is not subject to punishment for doing so. Another example is when a man goes to bed and finds a woman whom he thinks she is his wife and has intercourse with her; or when he calls his wife to his bed but some other woman instead comes for the purpose and he has intercourse with her on the thought that she is his wife; it could also be due to uncertainty as a result of blindness. In this case, he is by no means subject to punishment. 
There is a difference between zinā and a case where sexual intercourse takes place due to a mistake or to the fact that it is believed to be absolutely lawful. If, for instance, two sisters have been given in marriage to two men but each one of these sisters has slept with the wrong man by mistake, this cannot be considered an act of zinā because it is an intercourse which is believed to be utterly lawful; in case a child is born as a result of such intercourse, he is to derive his line of descent from the man who has fathered him, just like intercourse in an invalid marriage. Imām Ahmad said in this regard, "Anyone regarding whom punishment has been averted should take the child who will then derive his line of descent (nasab) from him."
Prohibition of Zinā
Zinā is an atrocious sin, a heinous crime, a pathway to evil, the cause of hate and enmity and a gateway to numerous diseases; indeed, it is for this reason that Almighty Allāh has prohibited it. He has even mentioned it a number of times in the Qur'ān to warn against its evils in different ways and various forms. Sometimes, He unequivocally commands us to eschew it and avoid all the ways that are bound to lead to it; sometimes He describes it as a foul, hateful and evil way; some other times He makes abstention from it a condition for taking the oath of allegiance ; on some occasions He describes the truthful believers as those who do not commit zinā; on other occasions He places it on an equal footing with associating false gods with Him in worship (shirk) and killing people without a just cause; sometimes He warns those who commit such a sin of doubling the punishment for them on the Day of Resurrection; some other times He plainly shows the severe punishment those who commit it should be subjected to in this life; on some occasions He commands the believers, men and women, to lower their gaze and guard their private parts; on other occasions He makes His slaves' salvation dependent upon their guarding their private parts; yet on other occasions He clearly states that adulterers and fornicators do not marry but adulteresses and fornicatresses or polytheists, and that such an abominable act is strictly forbidden to the believers. The following are ten ways of forbidding zinā and warning against it.
1. Unequivocal Warning to avoid all the Ways leading to it
Allāh the Almighty says, "And do not approach zinā; surely, it is an indecent deed and an evil way." 
It is clear in this verse that our Lord not only forbids us to commit zinā but also warns us against approaching it or engaging in any activity that is bound to lead to it. In fact, the expression used here is very subtle, for it seeks to close each and every door that is very likely to lead to zinā, be it a lustful glance or touch, a lewd gesture, an indecent word or licentious behaviour, an erotic story or scene, an attractive woman playing up her charms, captivating adornment, being left alone with a woman who is not an immediate family member and travelling abroad of a woman without a mahram , to mention but a few examples. All these things are bound to inflame erotic feelings and tempt one to commit the sin of zinā and immoral behaviour. It is for this reason, therefore, that Islam strives hard to eradicate seductions, close the door of temptation and sin and root out immorality and vice. 
2. Using the Worst Possible Words in its Description
Almighty Allāh says, "And do not approach zinā; surely, it is an indecent deed and an evil way."  Allāh here describes zinā using the word 'fāḥishah' (translated here as 'indecent deed'), which is derived from the word 'fāḥish' and means the 'indecent, ugly, foul and grotesque' deed which has reached the highest degree of obscenity and moral shock, thus affecting minds and the pure human nature. He also describes it as an evil way due to the countless evils to which it is bound to lead, such as working against the interest of the children, mixing up lineages, infringing upon other people's honour and inviolable rights, neglecting rights and obligations, destroying the basis of the family, loosening the bonds of society and causing murders and feuds and loss of reputation and property, all of which lead to too much evil and destruction. It is also an evil way due to the divine wrath that is bound to descend upon those who commit it and the fact that they become despicable and contemptible in the sight of Allāh and that they drop in the estimation of people who start to hold them in contempt. Besides, this sin causes a spiritual wickedness that is reflected in the face and heart of the sinner, leads to ignominy and lack of his sense of honour as well as to lying, infidelity and deception.
Furthermore, it causes fatal epidemic diseases, a decrease in one's worldly provisions, feelings of great unhappiness, deep embitterment and a whole host of other forms of punishments in this life and in the hereafter.
3. Making Abstention from it a Condition for taking the Oath of Allegiance
Almighty Allāh says, "O Prophet! When believing women come to you to take the oath of allegiance [at your hands] that they will not associate in worship any other thing whatsoever with Allāh, that they will not steal, that they will not commit adultery [or fornication], nor kill their children, nor utter slander, intentionally forging falsehood, nor disobey you in any just matter, then accept their allegiance and pray to Allāh for the forgiveness [of their sins]. Indeed, Allāh is Oft-Forgiving, Most Merciful." 
4, 5, and 6. Describing the Truthful Believers as those who do not commit Zinā, Placing Zinā on an Equal Footing with Committing the Grave Sin of Shirk and killing People without a Just Cause and Warning those who commit it of doubling the Punishment for them on the Day of Resurrection
Almighty Allāh says, "And the servants of the Gracious God are …those who invoke not, with Allāh, any other god, nor kill such life as Allāh has made sacred except for a just cause, nor commit adultery [or fornication]; and he who does that [not only] meets punishment, [but] the penalty on the Day of Judgment will be doubled to him, and he will dwell therein in ignominy…" 
This verse makes it clear that Allāh the Almighty considers abstention from the sin of zinā as a sign of the true believers and also places such a sin on an equal footing with associating others with Allāh in worship (shirk) and killing people without a just cause; He also doubles the punishment—for those who commit—in the Fire, where he will abide disgraced unless they turn to Allāh in repentance and works righteous deeds in this life.
Ibn al-Qayyim writes, "Because the evil of committing zinā is one of the greatest evils in that it goes against the laws of the universe as to preserving lineages, guarding the private parts, safeguarding one's honour and eschewing everything that may lead to the worst forms of hatred and enmity among people by leading one another's wife, daughter, sister or mother to the path of immorality, hence the moral destruction in the world, there is no other sin that may be considered next in enormity to that of taking people's lives which Allāh has made sacred than that of committing zinā.
Imām Aḥmad said in this regard, 'I do not know of any [sin] that is considered the next gravest sin after taking the life of a person which Allāh has made sacred than that of committing zinā.'"
Abul-Walīd b. Rushd also writes, "Therefore, zinā is one of the gravest sins and most heinous crimes; indeed, there is no sin after that of committing the sin of shirk and killing such life as Allāh has made sacred [except for a just cause] that is graver than it." 
7. Prescribing the Severe Punishment to which those who commit it should be subjected in this Life
Almighty Allāh says, "The woman and the man guilty of fornication—flog each one of them with a hundred stripes, and let not compassion move you in their case, in a matter prescribed by Allāh, if you believe in Allāh and the Last Day; and let a party of the believers witness their punishment."
8. Commanding the Believing Men and Women to drop their Gaze and Guard their Private Parts
Allāh the Almighty says, "Say to the believing men that they should lower their gaze and guard their modesty. That will make for greater purity for them. Surely, Allāh is well acquainted with all that they do. And say to the believing women that they should lower their gaze and guard their modesty..."
In this verse, Allāh commands the believing men and women to drop their gaze and preserve their chastity and then explains that doing so is purer for them and more beneficial for them in this life and in the life to come. Then He informs them that He is well aware of what they do. He also says in this connection, "He knows the treachery of the eyes and what the breasts conceal." 
Ibn al-Qayyim writes, "Due to the fact that [zinā] can be initiated as a result of a mere glance, Allāh commands [the believers] to lower their gaze before commanding them to guard their private parts; indeed, immoral acts take place as a result of a mere steady gaze, for, as the Arabic proverb goes, 'Fire begins with little sparks' . It all starts off with a look, which turns into a thought, then a step and finally a sin."
9. Making People's Salvation dependent upon their guarding of their Private Parts
Almighty Allāh says, "Successful indeed are the believers: those who humble themselves in their prayers; who avoid vain talk; who are active in deeds of charity; who guard their modesty, except with those joined to them in the marriage bond, or [the captives] whom their right hands possess, for [in their case] they are free from blame, but those whose desires exceed those limits are transgressors…"
Therefore, salvation can only be attained by guarding one's private parts against committing all that is unlawful. In fact, the last three verses here point out to three things:
Those who do not guard their modesty are not considered to be among the believers, that they are blameworthy as well as being transgressors.
Thus [worldly and eternal] success has eluded them; they become transgressors and deserve to be blameworthy.
10. Clearly stating that Adulterers and Fornicators do not marry but Adulteresses and Fornicatresses or Polytheists and that such Abominable Act is strictly Forbidden to the Believers
Almighty Allāh says, "The adulterer [or fornicator] shall not marry but an adulteress [or fornicatress] or a polytheist woman; and an adulteress [or fornicatress] shall not marry but an adulterer [or fornicator] or a polytheist man; to the believers such a thing is forbidden."
The meaning of this verse as well as scholars' opinions about it will be mentioned in due course.
Just like the Qur'ān, the Prophetic traditions are replete with commands to preserve sex purity, for men and women, at all times: before marriage, during marriage and after the dissolution of marriage in various ways and forms. The Prophet (ṣallallāhu `alayhi wa-sallam) said, "Whoever guarantees me [the chastity of] what is between his legs (i.e. his private parts), and what is between his jaws (i.e., his tongue), I guarantee him Paradise." 
Abu Hurayrah (raḍiyallāhu anhu) also narrated that Allāh's Messenger (ṣallallāhu `alayhi wa-sallam) said, "When an adulterer commits illegal sexual intercourse, then he is not a believer at the time he is doing it; and when somebody drinks an alcoholic drink, then he is not a believer at the time of drinking; and when a thief steals, he is not a believer at the time when he is stealing; and when a robber robs and the people look at him, then he is not a believer at the time of doing it." 
The Prophet (ṣallallāhu `alayhi wa-sallam) also mentioned in a long ḥadīth the punishment of those who commit the sin of zinā thus, "Last night two persons came to me [in a dream] and woke me up and said to me, 'Proceed!'… So we proceeded and came across something like a tannūr (a kind of baking oven, a pit usually clay-lined for baking bread) in which there was much noise and voices." The Prophet added, "We looked into it and found naked men and women, and behold, a flame of fire was reaching to them from underneath, and when it reached them, they cried loudly. I asked them, 'Who are these?' They said to me, 'Proceed!…" Towards the end of the ḥadīth, they told him about everything he had seen, thus, "…And those naked men and women whom you saw in a construction resembling an oven, they are the adulterers [and fornicators] and the adulteresses [and Fornicatresses]…'"  In fact, all Muslim scholars are unanimously agreed that zinā is strictly forbidden and that it is one of the major sins. Indeed, all the religions before the advent of Islam considered it forbidden and vilified those who committed it.
Sheikh Muḥammad ash-Sharbīnī writes, "All religions are unanimous as to the fact that [zinā] is strictly forbidden and that it is one of the worst of all of the major sins ; in fact, no religion considers it lawful, and it is for this reason that the punishment set for it is very severe indeed, as this [sin] represents a serious crime involving honour and lineage." 
 Surah an-Nūr :24:32.
 The Arabic word ayāmā (singular: ayyim, translated as 'those among you who are single') refers to anyone who is not in the bond of wedlock, whether unmarried or lawfully divorced or widowed. See Tafsīr Ibn Kathīr, 6/53-4.
 Taysīr al-Karīm ar-Rahmān Fī Tafsīr Kalām al-Mannān, 3/397.
 Surah an-Nisā, 4:3.
 Surah al-Baqarah, 2:232.
 Surah an-Nisā', 4:1.
 Surah an-Nahl, 16:72.
 Surah Yāsīn, 36:36.
 Surah ar-Ra'd, 13:38.
 Surah al-Furqān, 25:74. See Ihyā' 'Ulūm ad-Dīn along with the commentary Ithāf as-Sādah al-Muttaqīn, 6/12.
 Surah al-Mu'minūn, 23:5-6.
 The Arabic word 'bā'ah' (translated here as having the ability to marry linguistically means intercourse; scholars, however, have expressed two different opinions as to its technical sense. Some maintain that it denotes the linguistic meaning, i.e. sexual intercourse; others, however, hold that it refers to financial ability, named as such for being so closely related to marriage. This means, whoever has the financial ability to marry should get married, but whoever does not have it should fast. This interpretation of theirs is provided on the grounds that an impotent person does not need to fast to inhibit his sexual desire, hence their interpretation of 'bā'ah' as financial ability. An-Nawawī said in this connection, "These two opinions share the same meaning, namely, that whoever has the ability for intercourse on account of his financial ability [to provide the dowry, maintenance and carry out the other matrimonial duties] should get married; but whoever lacks such ability on account of his lack of financial ability must observe fasting in order to inhibit his sexual desire and put an end to the evil his sperm might lead to…" Ibn Ḥajar writes in Fath al-Bārī (9/108-9), "It is also possible that the word 'bā'ah' can be interpreted in its wider sense, namely, ability for intercourse as well as financial ability." See also Nayl al-Awtār, 7/259-260.
 The Arabic word 'wijā'' (translated here as 'shield') is a type of castration in which case the spermatic cord and blood vessels leading to the testicles are crushed or contused [to prevent blood reaching the testicles so that they gradually wither away and die.] What is meant here, however, is to inhibit the sexual desire." See Ibn Al-Athīr's Jāmi' al-Usūl, 11/428.
 Reported by al-Bukhārī, Book of Marriage, hadīth nos. 5065 and 5066, 3/354-355; also reported by Muslim, Book of Marriage, hadīth no. 1400, 2/1018-1020.
 Reported by Abu Dāwūd, Book of Marriage, hadīth no. 2050, 2/542 and An-Nasā'ī, Book of Marriage, hadīth no. 3227, 6/6566 on the authority of Mi'qal b. Yasār (raḍiyallāhu anhu).
 Some jurists have stated that marriage as the five categories of commandments and injunctions, namely, wājib (obligatory), (2) harām (unlawful), (3) mustahabb (desirable), (4) makrūh (undesirable) and (5) mubāh (permissible), depending on people's situations. For more details on this point, see Ibn Daqīq al-'Īd's Ihkām al-Ihkām, 4/22, Fath al-Bārī, 9/110-111, Subul as-Salām, 3/973 and Nayl al-Awtār, 7/2262.
 See Al-Hudūd wat-Ta'zīrāt 'Inda Ibn al-Qayyim, pp. 90-1.
 See Lisān al-'Arab, 14/360 and Al-Misbāh al-Munīr, 1/257.
 See Al-Mufradāt, p. 215; Mu'jam Maqāyīs al-Lughah, 3/26 and Al-Qāmūs al-Muhīt, p. 1667.
 The word 'ajnabiyyah' (lit. 'unrelated') refers to a woman over whom a man has no sexual rights since she is not his wife. [Translator's Note]
 Reported by Al-Bukhārī, Book of Asking Permission, hadīth no. 6243, 4/139; and Muslim, Book of Divine Destiny, hadīth no. 2656, 4/2046-7.
 At-Ta'rīfāt, p. 153; see also Al-Hudūd wat-Ta'zīrāt 'Inda Ibn al-Qayyim, pp. 92-3.
 See Badā'i' as-Sanā'i', 2/3311 and Hāshiyat Ibn 'Ābidīn, 22/258.
 Al-Mughnī, 12/344.
 Ibid. 11/171.
 Surah al-Isrā', 17:32.
 'Mahram' refers to a near relative with whom it is unlawful to marry. [Translator's Note]
 See Mushkilāt ash-Shabāb al-Jinsiyyah wal-'Ātifiyyah, p. 80.
 Surah al-Isrā', 17:32.
 See Al-Jawāb al-Kāfī, pp. 177-8 and Rawdhat al-Muhibbīn, pp. 360-3.
 Surah al-Mumtahinah, 60:12.
 For a full description of the characteristics of the 'servants of the Gracious God, Allāh', read verses 63-68 in Surah al-Furqān (Surah no. 25).
 Al-Jawāb al-Kāfī, p. 177; see also Rawdhat al-Muhibbīn, p. 357.
 Al-Muqaddimāt al-Mumahhidāt, 3/240.
 Surah an-Nūr, 24:2.
 Surah an-Nūr, 24:30-1.
 Surah Ghāfir, 40:19.
 This is equivalent to the well-known English proverb "Constant Dropping wears away Stones". [Translator's Note]
 Surah al-Mu'minūn, 23: 1-7.
 Al-Jawāb al-Kāfī, p. 178.
 Surah an-Nūr, 24:3.
 Reported by al-Bukhārī on the authority of Sahl b. Sa'īd as-Sā'idī, Book of Allāh's Limits, hadīth no. 6807, 4/2252.
 Reported by al-Bukhārī, Book of Allāh's Limits, hadīth no. 6810, 4/2252 and Muslim, Book of Faith, hadīth no. 57, 1/76.
 Reported by al-Bukhārī in different chapters; see for instance hadīth no. 7047, 4/310-1.
 See Al-Jāmi' Li Ahkām al-Qur'ān, 10/253 and Rahmat-ul-Ummah fī Ikhtilāf al-A'immah, p. 284.