When I was growing up, there was one rule guiding all sleepovers, and that was the gender one. If you were going to have other kids stay over in a sleeping bag on the floor of your bedroom, it was either girls or boys only. No mixing it up, no shades of gray.
The purpose of this rule was, presumably, to prevent kids from waking up in the middle of the night and spontaneously having sex with each other. That 8-year-olds don’t really do this was beside the point. Sex was very, very dangerous...