Minors, sex, and film making, art imitating life in Juxtaposition?

I heard once it is said of film as art imitating life, in some ways that do not apply at all. In film if you have an actor/actress who is a minor it is quite acceptable for them to pretend to kill, maim, rape and assault someone but not OK for them to pretend to have sex with anyone, especially an actor/actress older than 18. But flip the switch in the real world society is pretty much OK with minors having real sex just who they are having real sex with, and so long as they have a box of condoms and Planned Parenthood on speed dial its all good to go; but if they actually do main, rape, assault and kill we no longer want to treat them as kids but up them to adult status. Is it that seeing kids do violence in film is less ghastly because we know deep down it is just a show and none of it is real? If that is so if the story is like the Lover, Lolita(1997_film) or The Crush(1993_film) where the story line involves a precocious youth the pretend, not real sex is more squeamish to watch than gratuitous violence?