You know, I have trouble with this. I have known several girls who have gotten pregnant when their parents *allowed* the boyfriends to spend the night on a regular basis, versus those parents who didn’t allow this. I definitely think there is a correlation. Of course, significant others in general I would think could be problematic, but then how do you, as a parent, separate who is a friend, who is a significant other, and who is a “want to be more than friends” friend. I want to say not until they turn 18 and are an “adult”.
That being said, I often spent the night at male friends houses and vice versa. Usually we were just up late, someone laid down, and we fell asleep. There was maybe only one occasion when it was even sort of planned. My parents kind of had a problem with it, but only kind of, and the reason for that is because I was a good kid. I’ve never had sex, and the people who slept over were always very respectful and only friends.
Its a tough one.