I think the issue is that sex is intended to be the result of a mature and long-term involvement, which games don't really provide. Even if you made a game where you play as a married man making love to his wife, the gamer is still getting gratification in the short term - which is still better than most movies, obviously, but the issue comes with the players involvement. Maybe it's ok for Keanu Reeves and Sandra Bullock to bone after spending 90 minutes together on a speeding bus (pardon the reference but I caught Speed on TV yesterday and it was the first thing to spring to mind), but we're not involved in that decision. In games we are.
I'm not saying sex in games can't be done tastefully, or done well. They really can. I think Mass Effect was a good example, especially considering how cinematic the game was overall. There are many games with a strong story focus that can benefit from a love/sex component, as long as it doesn't become a crutch for storytellers every time there's a guy and a girl in the same room.
On the other hand, violence can happen quickly, unexpectedly, and all the damn time.