Sex is a sacred thing, which is why sex sins are “special” sins. Sexuality is a life function, like breathing. It serves the species. Therefore, conscience protects sexuality in a special way.
Mostly, God, through Nature, with conscience, protects sexuality by requiring that we hone to the pre-wired “shape” of sexuality – engaging in sex which is an expression of heterosexual union which does not interfere with that reproductive potential of the union naturally appurtenant to the heterosexual shape, for which sexual pleasure is the reward.
That one principle – the requirement that we bow down to any reproductive potential naturally appurtenant to the heterosexual union of the sex act – is the reason for almost all of the special moral requirements governing sexual relatioinships:
Loose sex is non-unitive, therefore immoral.
Sex protected against any existing reproductive potential by birth control mechanisms comprises an attack on existing reproducdtive potential to get the sexual pleasure for free, and is therefore immoral.
Sex engaged in without attention to the sexual pleasure of the partner – “taking” sex – interferes with the sexual pleasure “trade-off” in the heterosexual relationship and is therefore immoral.
But, we men are entitled to occasional freebees.
Just joking on that last one.