My theory is that the main difference between acceptance of sex and violence in the media is the "corruption of youth" principle. Look at the history of censorship in America and time and time again there have been crackdowns on movies, pulp magazines, comic books and video games justified by "what about the children?" arguments.
Why? Because while excessive violence might inspire violent acts in real life, those acts are already regulated and punished by law. On the other hand, sexual content and (consensual) sex acts depicted in a movie or video game could (oh, my gosh) inspire "immoral" conduct by our youth with no law to hold it in check. That's the fear.
The best example of this skewing of violence vs. sex is the teenage slasher movie cliche where the horny couple sneaking off to do their thing is almost guaranteed to be the next ones meeting the sharp end of a rusty implement.
Face it, we're a society that almost fetishizes innocence (moral, physical and mental) and the protection of it. Look at adamant reactions to "free range" kids, teaching of evolution or safe sex, and fear of vaccinations.
To be clear, as a parent I believe in protecting my kids' innocence as long as possible. I also strive to protect them from ugly or gratuitous depictions of any of the traditional sins including greed, gluttony, etc. But, when inevitably reality intersects their growing ability to handle it, I may cringe or cry a little but look forward to the day their innocence is no longer imposed on them but is something they value and choose.