Heard my brother having sex with his girlfriend?!?

Why would you slap him? He hasn't done anything wrong, he's an 18yo boy with a girlfriend, these things are going to happen. I had to deal with it too, and it's kind of awkward, but in the end it's not your business. If you can hear them and it weirds you out, just put on headphones or move to a different part of the house.