OP, I understand your points and your concerns, really I do. And for the most part, I agree with you. This "he's an adult" baloney is just that...a bunch of baloney. The kid is 18, in high school and probably couldn't support himself if his life depended on it...at least he couldn't support himself well. He is living in your house, with your rules.
As far as the sex part...well whether they are away at the beach house or in your hometown, the sex is going to happen. You can make it clear that you don't like it, you can think less of the girl because she is having sex with him...you can do all sorts of things but you're not going to stop it so don't waste your time even worrying about that.
Be upfront with the kid and say "Look, you know I'm not thrilled about this weekend away for many reasons. You know my reasons, we've discussed them enough. But I also know that you are 18 and as such, are legally an adult so I cannot tell you what to do. I know the prom is a special occasion and all, so while I'm not thrilled, I can't really stop you from going. That being said, I hope this sleepover stuff isn't going to become a habit because frankly, if ti does, I am going to assume you are quite ready to be treated more like an adult and we'll lay down some new ground rules in terms of what we will expect from our adult son as far as working, paying rent, taking over some of your own expenses etc". I'd also give the safe sex talk and remind him that a child is a forever commitment. I'd also remind him that if something did happen and this girl gets pregnant, that the annoying woman who is her mother will be in his life for the rest of his life too as his child's other grandmother. That may be enough to scare him if he really thinks about it.
Because really, if someone wants to be an "adult" in one area of their life, then they should be prepared to be an "adult" in all areas of their life. You can't be an adult for whooping it up with your honey at the beach house on the weekends and still be asking Mommy & Daddy for gas money to drive to there.....
As far as what your younger kids think or see or whatever....cross that bridge when you get to it. They may have completely different personalities and not want to be that involved with a boyfriend/girlfriend.