The word(s) "illegal" appears 42 time(s) in 36 verse(s) in Quran in Mohsin Khan translation.

(11) And whoever of you have not the means wherewith to wed free, believing women, they may wed believing girls from among those (slaves) whom your right hands possess, and Allah has full knowledge about your Faith; you are one from another. Wed them with the permission of their own folk (guardians, Auliya' or masters) and give them their Mahr according to what is reasonable; they (the above said captive and slave-girls) should be chaste, not committing illegal sex, nor taking boy-friends. And after they have been taken in wedlock, if they commit illegal sexual intercourse, their punishment is half that for free (unmarried) women. This is for him among you who is afraid of being harmed in his religion or in his body; but it is better for you that you practise self-restraint, and Allah is Oft-Forgiving, Most Merciful.

(12) Had not the Grace of Allah and His Mercy been upon you (O Muhammad صلى الله عليه وسلم), a party of them would certainly have made a decision to mislead you, but (in fact) they mislead none except their own selves, and no harm can they do to you in the least. Allah has sent down to you the Book (The Qur'an), and Al-Hikmah (Islamic laws, knowledge of legal and illegal things i.e. the Prophet's Sunnah - legal ways), and taught you that which you knew not. And Ever Great is the Grace of Allah unto you (O Muhammad صلى الله عليه وسلم).

(13) And because of their (Jews) disbelief and uttering against Maryam (Mary السلام عليهما) a grave false charge (that she has committed illegal sexual intercourse);

(14) Made lawful to you this day are At-Tayyibat [all kinds of Halal (lawful) foods, which Allah has made lawful (meat of slaughtered eatable animals, milk products, fats, vegetables and fruits)]. The food (slaughtered cattle, eatable animals) of the people of the Scripture (Jews and Christians) is lawful to you and yours is lawful to them. (Lawful to you in marriage) are chaste women from the believers and chaste women from those who were given the Scripture (Jews and Christians) before your time when you have given their due Mahr (bridal-money given by the husband to his wife at the time of marriage), desiring chastity (i.e. taking them in legal wedlock) not committing illegal sexual intercourse, nor taking them as girl-friends. And whosoever disbelieves in Faith [i.e. in the Oneness of Allah and in all the other Articles of Faith i.e. His (Allah's) Angels, His Holy Books, His Messengers, the Day of Resurrection and Al-Qadar (Divine Preordainments)], then fruitless is his work; and in the Hereafter he will be among the losers.

(15) And you see many of them (Jews) hurrying towards sin and transgression, and eating illegal things [as bribes and Riba (usury)]. Evil indeed is that which they have been doing.

(16) Why do not the rabbis and the religious learned men forbid them from uttering sinful words and from eating illegal things. Evil indeed is that which they have been performing.

(17) Say (O Muhammad صلى الله عليه وسلم): "Come, I will recite what your Lord has prohibited you from: Join not anything in worship with Him; be good and dutiful to your parents; kill not your children because of poverty - We provide sustenance for you and for them; come not near to Al-Fawahish (shameful sins, illegal sexual intercourse) whether committed openly or secretly; and kill not anyone whom Allah has forbidden, except for a just cause (according to Islamic law). This He has commanded you that you may understand.

(18) Alif-Lam-Ra. [These letters are one of the miracles of the Qur'an, and none but Allah (Alone) knows their meanings]. These are the Verses of the Clear Book (the Qur'an that makes clear the legal and illegal things, laws, a guidance and a blessing).

(19) And indeed she did desire him, and he would have inclined to her desire, had he not seen the evidence of his Lord. Thus it was, that We might turn away from him evil and illegal sexual intercourse. Surely, he was one of Our chosen (guided) slaves.

(20) Verily, Allah enjoins Al-'Adl (i.e. justice and worshipping none but Allah Alone - Islamic Monotheism) and Al-Ihsan [i.e. to be patient in performing your duties to Allah, totally for Allah's sake and in accordance with the Sunnah (legal ways) of the Prophet صلى الله عليه وسلم in a perfect manner], and giving (help) to kith and kin (i.e. all that Allah has ordered you to give them e.g., wealth, visiting, looking after them, or any other kind of help), and forbids Al-Fahsha' (i.e all evil deeds, e.g. illegal sexual acts, disobedience of parents, polytheism, to tell lies, to give false witness, to kill a life without right), and Al-Munkar (i.e all that is prohibited by Islamic law: polytheism of every kind, disbelief and every kind of evil deeds), and Al-Baghy (i.e. all kinds of oppression). He admonishes you, that you may take heed.