The Ways of Ascent - 70:30
Except for their wives, that is, those to whom their oaths/contracts entitle them. They will not be condemned.
Fornication is one of the great sins:
The Night Journey/17:32
Do not approach adultery, for it is a great sin and an evil deed.
وَلَا تَقْرَبُوا الزِّنٰٓى اِنَّهُ كَانَ فَاحِشَةًۜ وَسَٓاءَ سَب۪يلاً