I'll go devil's advocate here. Before puberty we really don't know/care about sex. Kids shown this picture only see dolphins:
It's as harmless an activity as playing soccer. They're playing dress-up and getting to pretend to be adults, something they'd be doing at home anyway.
When they're older they're going to be fucking like rabbits anyway, all teenagers do. As long as they're taught about contraception and disease prevention I don't see the harm.
The most disease and unwanted pregnancies / abortions happen where abstinence is promoted; the bible belt, catholic schools, etc. It's adult uncomfortability about sex that makes kids a) want to do it more and b) know less about protecting themselves concerning sex.
In the past seeing sex as sinful / bad was advantageous as it prevented disease and unwanted children. Now science can accomplish that goal while allowing us to have lots of sex. Religion and its stuffy attitudes are no longer necessary and it all has to go.