If people are scared about having boys and girls together you don't have to be at your house to do those types of things. You can do it in the bathroom at school, in the stairwell at school. It's not like the only time I'm going to see a boy is when my mother calls his mother and tells him to come over. Whether a kid can have a co-ed sleepover should have to do with the maturity level of the kid, not his or her age. Even if the kids know about the human body and knows what sex is, it not like they are necessarily going to want to experiment and have sex.
Sex is everywhere.
It's a common thing. I think adults always think of sex as a forbidden thing and teenagers as horny monsters. That's not always necessarily true. They also have sex because it feels good. However, I do agree that when you are 13, the one thing you should not be focusing or worrying about is sex.