My position is exactly like the one Wahine's ex had. She articulated it much better. It's research based. But, it's also how I really feel. Although most parents don't want to believe it, kids have sex! My mom was also very open to discussing this stuff with me (heck, it was the late sixties), and that was my model.
Lph, my son slept over a girl's house after graduation. They camped out in her backyard. Of course, he was 18 by then, but neither us or her parents thought anything about it. They were super friends all through high school and then went to different colleges in the same town (she went to Smith, a single gender school, and he used to visit her all of the time there) and remained great friends. I think he really liked her in a more serious way, but I never squashed any of it. Come to think of it, this same son had a mixed gender sleep over after the junior prom. There were 6 of them, who slept in our basement. They were couples, but my son's date was just a friend. They were 16-17 and we drove them to the prom, and they even called us to come pick them up early! DS was driving then, but the rules don't allow people under 18 to drive unrelated teens in the car. I made them French toast in the morning and it was no big deal.