In my opinion, three reasons (sorry if these were already mentioned, I just want to get my ideas out before reading through the thread):
First, society. It's been mentioned a lot that as a western society we're more open to violence than sex. Blame America's puritan roots. Blame conservative thinking. Blame the overly zealous Christian groups that say if any adult content is in anything the children will automatically see it and they'll all become gay satanists who have blood orgies while shooting up heroine.
No matter who is blamed, it's true. We're just not open to sex as a society. Implied sex is a norm in other media. But having a woman in her underwear in a picture or on TV is different than having an interactive woman in her underwear.
Second, games are still thought of as toys and violence (outside of gratuitous violence and gore), being more accepted, is seen as a natural part of play for boys and teens and younger men. I mean, sports can be violent and even when no one gets hurt a lot of sports are more than happy to let people bash into each other for the sake of the game. Or you can look at the old go to Cops and Robbers/Cowboys and Indians.
Third, and probably the biggest reason in my eyes, violence can move a story forward with interest. I can be told that the bad guy is hiding in the next building and I need to eradicate them. So, in gameplay, I go over and eradicate them. Easy, yes?
Sex can't be treated so easily for a number of reasons. It can't be used as a gameplay element or it turns into Hot Coffee. It can't generally be used to advance the story, because let's face it video games don't take a mature enough approach to use it as integral part of the story. It can't be too detailed, as that would spark controversy as well, but making it too ambiguous causes it to lose it's power from a storytelling aspect. It can't be taken too lightly as most people can see how sex is ingrained in our minds. Sex for most things is procreation, for us it's procreation and recreation and emotional attachments and all sorts of things that make sex more than just a single act of release or mating. But too seriously and they call is a sex simulator.
Mass Effect did an alright job with sex, though, Dragon Age did a better job in my eyes (because after having sex with a character it changed the relationship no matter who it was with, giving sex a deeper meaning).
While God of War, for instance, does sex in all kinds of wrong.
But the easy answer?
Sex is powerful, violence is easy.