Here's a few reasons I can think of, at any rate...
1) Sex is part of a "mature" game. You can become involved with the story on a personal level and the romances feel a bit more in-depth (I'm not saying romance is all about sex, don't set fire to that particularly flammable statement).
2) I think it was Eric Bischoff who said "Controversy creates cash". The man's a loon and an all around lesser human being, but the statement has some ring of truth about it (especially when you look at the huge scandal - and subsequent sales figures - of Modern Warfare 2's opening scene, which despite being skippable and not actually having to do anything is STILL being attacked as some kind of Terrorists'R'Us simulator).
3) Bioware obviously noticed that it was a popular addition with the original Mass Effect and it's only snowballed from there. Bioware love to skim their social forums and romance has become a big part of their "game template".
4) If it isn't in the original games, mods will come out making versions that are ten times worse. If those get to the media, it doesn't matter whether or not it's user generated content or not. Shitstorm ensues. I'm not saying that players won't still make nudity patches, cutscenes etc (hell, I'd be surprised if they didn't) but having it there to begin with stops it from circulating around some of the more... frisky... playerbase and thus decreases its chance of coming up around the media.
5) Could be in character? Hell, I always play a chaotic neutral absolute bastard who's a good guy (very deep) at heart. Games like Oblivion wouldn't let me show that, but games like Dragon Age (despite the whole "you're a Gray Warden, go there do this save the world" thing) allowed me to show what I want my character to say and do because in a Roleplaying game, I want to play a role (crazy, I know). Hell, I had a good time IN DEFIANCE of the Gray Warden thing, being the roguish spoiled nobleman who ends up being a sulky sod when his family dies and he's one of the only two people who can sort stuff out, and then grows the hell up when he starts interacting with Leliana and figures that other people have had it just as bad.
6) Gamers are lonely, horny individuals.