I'm confused about several things. These kids - your daughter and her boyfriend - are legally underage, yet they are having sex. Then you say "they would not be able to share a hotel room or just go off and disappear together". Then where, I would ask, are they having sex? In your living room? It seems as though they're going off and disappearing together now, and having sex. Where else could they possibly be having sex?
Yet you seem worried about a precedent being set for your other children. You want to reserve the right to determine whether a relationship meets your standards for maturity before allowing a guest to be invited.
You've raised a girl who's been having sex at age 17 with a boy she's been dating for a year. Those are the precedents you've established. Condoning an action means that you don't stop it, you don't allow it, you know it's happening. You are condoning sexual activity between very young teens and that's your playbook now.
If you choose to allow a boyfriend or girlfriend to accompany your family, you've got to make some standards of your own. Perhaps you will decide if the dating relationship has gone on for 6 months, the date can come along. Maybe you'll decide on how many breakups and makeups they've had.
You've allowed friends to come along in the past, so that seems to be the precedent you've established. I would not have allowed a sexual relationship to persist when my child was this age, but if you decide to do that, then you've got to relax your other rules too.