With DS and his large group of friends we hosted the first mixed sleepover of the group, when they were 15 or 16 (I can't remember which). DS wanted to invite all his friends, and most of them lived too far away to get back easily late at night. DS is extremely sensible, and so are all his friends - they are all studious nerdy types.
I felt a bit uneasy about it but said it was ok, as long as they followed my rules, which I went through with all of them there. So there was a girls' room and a boys' room, and they weren't allowed in the other room at any time. They had the sitting room to themselves and had to have a definite bedtime, agreed with me in advance. DS had the choice of enforcing it, or having me come down in my nightie to enforce it - oddly, he chose the former.
I had a wander around the house about half an hour after they had gone to bed, and they were all crashed out in the right rooms.
The good thing about it was that because we had the first one, all the other parents have followed our rules about separate rooms for sleepovers. We've probably hosted seven or eight over the last couple of years, more than the other families, so they can't have had too horrible a time under my dictatorial regime. As they have got older they have gone on holiday together, first with one of the families, and then on their own. They are all now legally adults - but the rules still applied at the last pre-university sleepover we've just had.
Having said all that, my other DC can be much more flighty, and I may well not allow them to go to mixed sleepovers when the time comes - given I feel they are much less trustworthy than DS1 at the same age.
What did shock me about the first sleepover was that only one mother checked directly with me, and you could hear the relief in her voice when I said that DH and I would be here all night, and that I was going to strictly enforce separate rooms and a definite bedtime. I think some of the parents were probably reassured because DH is a vicar!
So, to summarise - I never thought I would agree with mixed sleepovers, but with the right children and the right rules it can be OK. And I would rather host them than send my kids off to them, at least with parents I don't know.