I only have a seven year old, so as a parent I can't comment too much,
But my teenage years aren't that far behind me, my mum never let my boyfriend stay over until I was already pregnant and there was a rule of no sex in her house, which I always thought was quite weird- we were doing it everywhere else anyway!!
I think as long as they remain respectful to you, and the other DCs- ie no loud sex, or staying in bed humping all day then there isn't really a problem is there?
I do know that in Holland they have a very different view to us Brits, they are really quite open with their teens about sex, and allow this kind of sleepover, and they have much, much lower rates of teen pregnancy, so it might be the way to go.
What age is your daughter though? I do wonder, because (whilst it wasn't a concern to me or my parents as a teenager) I wouldn't feel half as comfortable with my DD having a BF stay and possibly be having sex with until the age of consent. I just wouldn't want to be viewed by my child as someone with little regard for the law.
I wish you luck on this subject.., I'll keep following because this is an area I think I would have kittens over when DD is a teen... I'm kind of scared already!!
goes too to start building DDs Ivory tower.....hoping it'll be built in time!!