For some reasons parents don't mind violence,
We are born selfish and violent, lashing out (stomping feet, hitting, biting, scratch, hitting, etc) when we don't get want.
but show one schlong or some boobies, and that makes the movie off limits.
OTOH, we don't even start to become sexual beings until the early teen years. (Later, in cultures that aren't so sex-saturated as the US.)
Pretty screwed up world we live in.
Well, yes, but not for the reason you think.
You've never been outside the US then have you? England after watershed. The rest of Europe all the time. Nudity is kind of a fact of life everywhere but the censor happy US....
No amount of careful planning will ever replace dumb luck.