You should probably do your research.
There have been numerous Psychological studies throughout the years (none made real public because it's nothing "new") but it states that it's BETTER for children to have some sexual knowledge at younger ages
When kids start asking questions about their "parts" or about sex in general it's better to answer truthfully & not use terms like "pee-pee" or "down there" as it will cause confusion later on whereas telling them "You're a boy, you have a penis" or vice-versa for girls is better for their mind
Kids that commit more sexual deeds as a teenager usually is because their questions were never adequately answered by their parents so that they fully understand not only what their answers are, but the magnitude of whatever it is they are asking about so they are forced to rely on media and their friend's knowledge, which depicts mostly everything sexual as "good" so they see no issue in it
So in reality, it's the parent's fault for not educating their children about sex that they commit sexual acts at an "inappropriate" age. (whatever age you decide to be "inappropriate")
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Back on topic, though
I believe that, in general, Violence is indeed more "acceptable" than sex in the United States (see above for some reasoning) because Violence is straightforward. The repercussions are straightforward and there is no good/bad line with it
It's just "bad" according to the general society.
Sex, however, has standards of when it is Ok depending on factors like age, who your having it with, the sex of that person, the race of that person in some situations, and the emotional attachment.
Since sex is a lot more complicated than violence, using it in things like Video Games like a straight-up sex scene would then require the kids who play the game to understand the reason it might be Ok in that situation
Which would then require the game makers to be responsible in their depiction of the sexual act, otherwise they will get complaints up the ass for years to come if they use it wrongly