The Ayahs that state the prohibited sexual practices
A:
The Ayah that prohibits sodomy and masturbation, which is known as "the secret habit", is in Surah Al-A`raf where Allah (Exalted be He) says: (Part No. 22; Page No. 33)
And (remember) Lût (Lot), when he said to his people: "Do you commit the worst sin such as none preceding you has committed in the ‘Alamîn (mankind and jinn)?
"Verily, you practise your lusts on men instead of women. Nay, but you are a people transgressing beyond bounds (by committing great sins)."
Allah (Glorified be He) also says:
And those who guard their chastity (i.e. private parts, from illegal sexual acts)
Except from their wives or (the slaves) that their right hands possess, - for then, they are free from blame;
But whoever seeks beyond that, then those are the transgressors;