Both are regarded as if they should only be visible and exposed to adults. However, it is perfectly normal and natural for kids to want to have sex as soon as they hit puberty. And what harm are they doing if they do it safely anyway?You don't think the US should set the age of consent at 18, and you said "I never really understand this ridiculous idea that "sex and violence" should be handled in the same way."
How are they being handled in the same way?
For me, there is a much greater need to control exposure to violence in film (or in real life) until a much later (adult) age.