I do not have kids so I cannot really help much here. But, when it comes to sleepovers:

I am a woman but I have always had many male friends. My parents had the same attitude to it, the only boy they'd let sleep over was my friend whom they knew was gay. It's their house, so their rules apply, but I still found it very silly. Just because you are straight it doesn't mean you are interested in everyone of the other sex, and just because you are gay doesn't mean you are interested in everyone of the same sex. Frankly I think I only on a couple of occasions, and that was in later years (am 23 now), have at any moment felt a sexual attraction towards a friend. After someone has become a friend, it would take a lot for me to look at them in another way.

And, as my friend's mother says it (she's always been very tolerant about these things, even though she got pregnant early herself): Anything you can do in the night, you can also do in the day - before parents come home, at school, etc. It's not like it being night magically means you will act all different.