I do disagree because the drinking age where I am is 21, and drinking and driving is dangerous. So that would be asking if they can engage in illegal and potentially deadly activities. My kids will tell me if their friends drink at a party but they don’t, and they have all been comfortable with that too. But yes I like that they can tell me and ask for advice.
Sex, I very much prefer they have a safe space, I will leave and give them privacy, they are not doing anything wrong or illegal at all. I don’t actually think i’s a bad idea, it’s a natural part of a romantic relationship, and the kid is 17. And don’t understand your view that it’s safer for them to hide it from you? I hear the stories my kids tell about other kids and remember growing up in a household like that.
I guess we just disagree then. I literally don’t think it’s misbehaviour or anything to police like that. You want to put some sort of barrier up, but for what? So they will only do it at the girlfriend’s house? What does that accomplish? OP is not proposing to “provide turn down service, mints, and send them up to bed after dinner” just okaying a sleepover.