It’s a tough one. Think depends on child.
I was 24/25, living with boyfriend of about 4 years, and we still had separate rooms at my parents’ house, so I thought I’d be relatively strict. DH’s parents were the other extreme, so we agreed we’d probably be somewhere in the middle.
DS got a a girl friend in y12, and suddenly she stayed over in October of y13 when we went away with youngest. DC2 was very loyal to sibling and said nothing, but they forgot we had a ring doorbell. I suddenly thought: they are both nearly 18. Very serious, very conscientious. (One going to Oxford, one going to Med school.)
When we came home, I just said - we saw X stayed over on Ring doorbell. I am fine with it, I just hope that X’s parents did not think we were home and chaperoning, it is really important to be honest with everyone. I added that I hoped they are both being really careful, the pill and condoms, belt and braces as I am really not up for looking after a tiny baby.
I don’t think I’d be OK with DS bringing a different girl back every weekend - but it’s not about the sex, it’s about respect for his partner, and respect for the other members of his family (incl 2 younger sisters) and this works for us.