Interesting responses. I like reading your thought processes behind this.
I, of course, allowed this, LOL. I mean, challenging kid was 20 when he finally got his act together enough to move back home. He was respectful, helpful, doing things to move forward in his life. I remember what it's like to be 20.... so, yeah, overnights were allowed.
Now in my typical see-saw fashion, the party line as the kids were growing up, for *all* of them, was no sex before marriage. Not based on religious beliefs, but more on safety issues, avoiding unplanned parenthood, and giving my kids an "out" ("Mom said no sex before marriage") if they wanted/needed it.
But I am a realist, especially when it comes to challenging kid. He was staying in the basement when he came home, and the 2 younger kids were upstairs, so there was a geographical separation. The girls (okay, women) he had stay were always ones he was in a relationship with (I couldn't have dealt with a steady stream of strangers), and he *always* asked beforehand. The "rule" was that the only people allowed to have sex in my house were hubby and me. Do I think challenging kid complied with that rule? Nah. Did I expect him to - a 20/21/22/23-year-old? Nah. Did I want to avoid awkward naked encounters and unsettling sound effects? You bet. Has he had sex in my house? I have no proof of it, and that's the way I want it. Of course he did, but... I'm blissfully unaware.
Middle son hasn't had a date yet - though I finally did grow a pair and tell him that at age 21, it was time for him to move out of my house and start living his own life. Plus, I couldn't stand that he would just not go to work, not call in, but not get fired. Drove me CRAZY. So he's moved to a new time zone, and maybe he's had his first date by now, or more, but.... if he were to call and say he's coming home to visit and bringing Charlene (or Fred - who knows) - fine by me. Same rule applies. Can sleep in same bed but "no sex".
Ditto daughter who is now 18 and at college. Hubby would have a cow and I'd probably have to restrain him, but I don't believe in double standards.
Sex is part of life, in my humble opinion. Yes, I actually do wish they'd wait until marriage - not out of any moral thing, but more for the gift of giving oneself. But... that's my hindsight talking. My kids are actually much more hung up on sex and nudity than I ever was (the perils of having a mom who grew up in and embraced the 70's???) but... so far, they've all been responsible and respectful and smart - even challenging kid so.... yeah, I don't have a problem with overnight guests.