Alpha Protocol did it pretty well. Not to mention that this sort of thing (I wouldn't call it romance, really, not after the table scene with SIE) is espionage classics.
In DA II romance was ok, although everyone and their grandma trying to hit on me was freaking me out a bit at the beginning. Still the characters are likable and I can image all of them being popular romace options with some player niche.
Sex scenes in Witcher 2 (most of the romance there is implied, not roleplayed) seemed quite tasteful and fitting to me. I don't mind sex scenes as long as they are not too graphical and don't go all IKEA style. It is much better than a stock blackout screen.
That mentioned, I think romance should be more about relationships with the character (like it was in BG2 or PST) than actual sex. It should make NPC behave differently, say and do diffenrent things not only in the designated dialogue, but otherwise too. So introducing the sex scene as the only reward for having the romance at all (yeah, talking about Mass Effect here) feels extremely cheap and anticlimactic.
Love is about emotions. When it comes to that the neck-biting dialogue with Annah in Planescape: Torment has more weight than all romances from all ME and DA games combined.