Well, my kids never asked for any mixed gender sleep overs, as they were quite tame in HS. But, several times, I came home to find DS #1 "friend-girl" sprawled out on his bed with him, not really in a sexual way, but I am sure most parents would have freaked. My feeling is, if either of them had had a serious girlfriend at age 17-18 (senior year?) and I knew or suspected they were already having sex, I probably would allow it, of course, only after assurances they were using birth control. I would have no issues with helping in that, if the girl's parents wouldn't help. I also would have no issues with a separate bedroom sleep over.
I know a lot of parents would think this was awful, but I'd rather have my kids see me as an ally at that age. I would not treat a girl differently, either. I was, let's say, wild at a young age, and although my parents were very liberal, I am not sure how they would have felt about this. But, my mom did take me to get a BCP prescription when I was 17, under the guise of it being for "menstrual problems."