Lost it’s the “confined space” that’s a key part of the problem.
Anyone who is uncomfortable with what is happening to them, in the dark, has nowhere to go without making a big scene and waking everyone else up. And there might be hours until morning.
There are all sorts of barriers to saying “no” and defending your personal boundaries in this situation.
During the day you can far more easily walk away, find a public space, find an adult all without having to make a scene and humiliate yourself.
Much much harder if getting away means waking everyone up, waking an adult up.
At 11yo walking into a friend’s parents room in the middle of the night for help? That would seem almost impossible for lots of kids.
Imagine being an 11 year old lying next to someone in a dark tent who decides they want to touch you or kiss you in a way you don’t like, or show you something you don’t want to see?
Or starting off liking what was happening and then changing your mind because it’s gone too far?
How do you get out? How do you stop it? In the dark, surrounded by sleeping friends?
And this isn’t a boy thing or girl thing. I have one of each. I can see that they’d both be vulnerable in that situation.
Given our societal norms my son might actually be more vulnerable.
How would most Mums react if my ten year old son (who is taller than me) woke them up in the middle of the night because he couldn’t get a girl to leave him alone? They’d probably laugh right?
The reason I know lots of women would laugh is because that’s what happened when a group of girls deliberately tripped DS up and pinned him down at a party. For kisses.
He was horrified but couldn’t get away without hurting them. Their Mums thought it was “funny” and “cute”. 
Right up until I asked how they’d have felt if they situations were reversed at which point spluttering apologies were produced.
Not approving of mixed sex sleepovers isn’t about “hating evil boys”. It’s not about thinking all 11yos are up to no good. It’s not about thinking they are going to have sex.
It’s about not putting my children (male or female) in situations where they might be vulnerable, where they can’t easily defend their boundaries, where it’s harder for them to be able to assert their right to privacy.