I finally get a little internet time and this is MY opinion on the subject:
In game, my group tends to regulate the subject to a simple yes/no when it comes to the subject.
HUMAN Fighter : I'm trying to seduce the hot elven chick in the corner.
DM : Charisma check please.
Fighter: ......22!
DM : She spits in your face and kicks you in the junk. Roll a fortitude save.
Fighter : ....8.
DM : You fall the ground in agony as everyone points and laughs.
Fighter: But I rolled a 22!
DM : Maybe she hates humans, ever think of that?
ELVEN Ranger : I go over and lay my best lines on her. 13.
DM : She seems interested, but after a few minutes a really big elf dude comes out of a room behind her, sees yoou, and proceeds to try to strangle you.
Ranger : OH CRAP!
etc. etc. Occasionally someone gets lucky and it's a simple : You wake up with a smile on your face. Tada.
Venerial diseases can be fun to throw at your players as well. One time one of the PCs, a half dragon, got busy with another half dragon, and woke up the next day with a burning itch in their crotch. The cure happened to be plucking out his pubes one by one with a pair of tweezers and sewing them back on. He was quite abstinent afterwards.
Sex in D&D is what you make of it. If you have a mature group you could implement it via romances and create new plot hooks and character bonds, etc. Yet for most groups, it seems to be avoided or limited to brothels and one night stands.
Diff'rent strokes for diff'rent folks.
Yes, that is a pun. 