That's exactly the point. Violent behavior is in the realm of fantasy for most people as it is. The average person (at least the average middle american person far removed from the hood) is never really going to be able to simulate what goes on in the boxing matches, shooter games or horror movies in their lifetime. In turn it's easier (for most Americans) to pass it off as just entertainment. Further, even if a person has violent tendencies, engaging in those forms of media can work as a release valve so that you don't have to do it in real life. But sex is a lot more easily accessible because your'e already born with the impulse to do it and the only way to deal with those impulses is to either engage it head on or suppress it. In the psyche of the american public consciousness which is deeply rooted in puritanism and religion, sex is the ultimate temptation against their perception of morals and beliefs. It's more taboo, and taboos are inevitably going to be enticing to break. It's how the contradiction of Americans being absolutely obsessed with sex and pornography but at the same time being disgusted when two real people are actually doing it came about. The threat of someone being exposed to a sex scene at a young age and then growing up to become a lascivious pervert seems a lot more realistic to parents who've probably had to closet or fight off urges of their own than imagining their lil jimmy shoot up an airport because he saw it in Call of DutyBut it's true
Your children most likely won't grow up to be murderers but they will have sex at some point in their life most likely.
But look at which one people put more work into hiding from kids