Okay, here's the issue as I see it. The games industry is still driven by America to a large extent. America is a post-christian nation with a history of conservatism. Europe, you guys have been more open for a while...America still isn't.
The values that have driven this attitude toward Sex, Violence, and Language in media are a result of the largely christian history of the nation.
In the bible, it is admonished to control your tongue (words/swearing), to not engage in oppressive violence, or violence for the wrong reason (I would contend Jesus calls christians to pacifism), and to abstain from sexuality until marriage.
Now, why do we as a culture shun sexuality as opposed to violence? And why are we more likely to allow our children to see Lord of the Rings over a similarly rated PG-13 movie with language and "dirty joking" or the occasional breast? I believe it's actually some strange, subconscious equation that we do, which takes the Desirability, Ease of Access, Consequences, and other factors into account.
Violence, first of all, is not necessarily something that I would call easily accessible. Sure, you could pick up a rock and try to brain someone on the side of the road for no reason, but there is no motive, no reason to do it, and the consequences are heavy. Also, children and people do not usually behold violence of this type. Usually we are taught that violence is something to be used for good and not evil, and that there is a difference between murder, selfdefense, and war.
Desirability is low (who needs to kill anyone).
Ease of Access is moderate.
Consequences are high.
Language is another matter. There are no real consequences (besides a mouthful of soap), it isn't necessarily desirable or undesirable, but it's easily accessible. So parents find the action undesirable and easily reproduced. They make it a goal to not let their kids hear swearing.
Desirability is variable.
Ease of Access is high.
Consequences are minimal.
Now sex is something different. In America, shaped by christian values for so many years, sex is viewed as something done in marriage. It is viewed as proper in such a context, and while these views have been weakening for years, we still have many conservative people who would rather not be exposed to sex. Sexuality is extremely accessible. Not necessarily intercourse, but thoughts, porn, and "other outlets" (that parents and others find undesirable).
Sex is also viewed as having greater consequences than most secular people realize. In the bible it is mentioned that a man and woman experience a connection so deep and intimate, they are said to be "one flesh," or to become one. Also, fornication is looked at negatively in the bible, but more importantly, basically since all of history ever. But after these possibly perceived consequences(I've heard lots of regret from people with multiple sexual partners before, so maybe there's some validity) come the real consequences. Pregnancy, STIs, and social stigmas are all a part of the deal, and parents especially want to shield their children from them.
Desirability is high (feels good right).
Ease of Access is high(ish, depending on values).
Consequences are (potentially) high.
That is why I feel there is a divide between sex and violence in media representations (with a nod to language). Violence can be considered good by the old crowd. Sex (outside certain parameters) is seen as bad in all but one circumstance. The old crowd is large, and made up of middle aged americans with kids and jobs who have taken some of their parents values.
They have the money.
The next generation will likely look much different, so don't bemoan your circumstances excapists! Make the world what you want it to be!