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

(31) If the hypocrites, and those in whose hearts is a disease (evil desire for illegal sex), and those who spread false news among the people in Al-Madinah stop not, We shall certainly let you overpower them: then they will not be able to stay in it as your neighbours but a little while.

(32) And those who avoid the greater sins, and Al-Fawahish (illegal sexual intercourse), and when they are angry, forgive.

(33) Those who avoid great sins (See the Qur'an, Verses: 6:152,153) and Al-Fawahish (illegal sexual intercourse) except the small faults, - verily, your Lord is of Vast Forgiveness. He knows you well when He created you from the earth (Adam), and when you were fetuses in your mothers' wombs. So ascribe not purity to yourselves. He knows best him who fears Allah and keeps his duty to Him [i.e. those who are Al-Muttaqun (the pious. See V.2:2)].

(34) O Prophet! When believing women come to you to give you the Bai'ah (pledge), that they will not associate anything in worship with Allah, that they will not steal, that they will not commit illegal sexual intercourse, that they will not kill their children, that they will not utter slander, intentionally forging falsehood (i.e. by making illegal children belonging to their husbands), and that they will not disobey you in Ma'ruf (Islamic Monotheism and all that which Islam ordains), then accept their Bai'ah (pledge), and ask Allah to forgive them. Verily, Allah is Oft-Forgiving, Most Merciful.

(35) O Prophet (صلى الله عليه وسلم)! When you divorce women, divorce them at their 'Iddah (prescribed periods) and count (accurately) their 'Iddah (periods ). And fear Allah your Lord (O Muslims). And turn them not out of their (husband's) homes nor shall they (themselves) leave, except in case they are guilty of some open illegal sexual intercourse. And those are the set limits of Allah. And whosoever transgresses the set limits of Allah, then indeed he has wronged himself. You (the one who divorces his wife) know not it may be that Allah will afterward bring some new thing to pass (i.e. to return her back to you if that was the first or second divorce).

(36) And those who guard their chastity (i.e. private parts from illegal sexual acts).