Hey Suseyloren
I'm probably the odd one out again, but I don't have a problem knowing my son is having sex. He's 19 and his gf is a little older. They even stay at my house.!! oooooooh lol but..I can't be hypocritical about it.
When my husband and I were dating, of course we did the deed, and the bad part about it was that we had to hide, and always lie and pretend we weren't doing any such thing.... and we ended up in some really raunchy and, dangerous places.
I didn't tell my son to go ahead and have sex all over our house, but I just said 'you girl friend is a nice kid, and if you want to bring her here and you guys want to hang out in your room that's ok with your dad and I.' and it just made everyone relax,
they're here a lot, and we got to know her really well as a result. which was nice.
So this way there is no wondering where they are, and we know they're safe. His gf lives so far away, and during our really bad winter months I use to worry about her getting home as well so this worked out where she didn't have to leave in the middle of the night during bad ice and snow, and he didn't have to leave her house either to drive home.
It's your house to lay your rules, but you can't stop them from being intimate, and if it were me, I'd be worring about where the heck they were doing it. Not to minimize your concern at all, but they really could be doing worse things. JMO
Good Luck