It's society. And this explaination follows a very windy road, so you'll have to bare with me as we walk down it.
In America, we're based on religion. In Washington (DC), you can't get very far with public voters if you're not openly faithful. Politicians are poo pooed on for not going to church. In extension, people have demanded (in that collective fashion) for years that society has been losing its grip on moral fiber. Here's the groundwork.
Television and radio, the first forms of entertainment, had standards and practices that they adhered to. At first, it was no swearing in radio. It didn't take much to push the envelope. Television entertainment was pretty well the same. Then society started to change, wanting more. (I would say Rock and Roll and Elvis had a lot of influence here.) We went from black and white car chases, tense moments, and small on screen out bursts to upping the ante.
Over the years, television has slowly become more and more riske. With the original A-Team, you had action and violence, but NO ONE ever died. No one was ever seen dying on screen. If they took a bullet, it was a flesh wound. Skip forward to 24, and people will get shot on screen. We'll see blood. That would have been unheard of 40 some years ago.
Sex hasn't remained unchanged in television cerca 1940's. It's been expanding slower than violence. Remember that religious background Washington has been seeping with? Well in the bible, sex is a sin. For whatever reason, it's a bigger sin than violence (there's a lot of violence in the bible). That's one aspect that's hung around. In modern television, we have sex scenes that are hinted at, never seen. (Cable programs will take it to the level of soft core porn.)
Society has slowly become more and more desensitized to entertainment, requiring more envelope pushing extremes. It's always had standards and practices that it wasn't allowed to breach, but society has slowly changed to expect it, even demand it. Which is why if you ever watch television from the 70's and 80's, it seems so dull. It wasn't the same standards, and it was far more about the expectations of society in that era.
What does this have to do with gaming? Well in Television, there were standards that were originally picked up from radio. Gaming likewise took ques from television. Radio, television, and gaming have all evolved in their own ways, and there is sex in all of them now. But there is still the boundries of today that expect that sex will be limited, because we (rather, those that are in power that are expected to adhere to the practices of Faith and the Bible) see it as a greater sin than violence. Entertainment went the way of the action hero, and evolved. We evolved with it, and now it's nothing to us to see someone get their brains blown out on screen. Since sex didn't have the same opportunities, it's a bit behind the times.
Sex is catching up, in gaming, though. I think you can thank rating systems for that too. There are (mainstream US) games specifically for the male gamer to watch and interact with nude women. BME XXXtreme was one that came to mind. (I never played, but since it's not on some holy grail, I can only imagine its not terribly good as a sexual experience). It's just going to take some time. I imagine in 2020, we'll be leagues ahead with sex in our entertainment (not talking about porn).
Bioware has been introducing same sex partners into gaming experiences. Yes, the sex is still very PG-13 at best. But its still a huge move forward in gaming and sex in entertainment. Eventually, we'll reach a point where M-for-Mature will be able to include the same sexual content as R-rated movies. Which is the occational topless woman.
In the meantime, other countries have developed differently, and their sexual evolution in entertainment has too. Countries like Japan have sex integrated into some games moreso than violence. You'll have to get your fix there, for now. That is until the expectations of society as a whole for entertainment demands that little bit more.
