My uncle back when I was really young showed me and my cousin "Full Metal Jacket". I still remember some scenes, the film traumatized me, literally. I don't know what age I was but I was way too young for that.
Kids can get terrified by horror and violence. Sex? Not so much. I don't understand why we hide nudity from kids.
Because we are afraid of harming them, because we are conditioned to be afraid of ourselves. !Kung Bushmen often lived in hand-made temporary huts. Therefore it was basically normal for couples to make love in front of the kids. !Kung Bushmen brides were also once prized by neighboring agricultural people, as they were thought to be particularly uninhibited and enthusiastic sexually.
The US is still very much a puritan culture in some weirdly sublimed ways.
Making love in front of someone is different though. I think it's for the same reason someone doesn't like to be sober around drunk people, or the reverse.
I got "traumatized" by Baywatch (something with a ghost on the beach) and the short (2-4 sentences) teaser texts of mystery/horror-books in a book catalogue (with very broad sortiment) that came to our house.
On the other hand I coped well with Mad Max and Missing in Action. You just can't predict what hits.
Kids can get terrified by horror and violence. Sex? Not so much. I don't understand why we hide nudity from kids.