I stay up late to listen to my roommate and his gf.
Why are you snooping on them? Is this something sexual?
Yes, I can hear them having sex a lot, his gf is a little slut
Isn't that a bit disgusting and annoying to hear them having sex? You should tell them that you can overhear the sex so they are aware and stop being so loud and respect your boundaries. It's rude and unfair to you as a roommate and you shouldn't tolerate the behavior because it might make you depressed for not having a girlfriend.
But they are always so loud when they are together.
Spying on people is still wrong. I think it might be time for you to find a new roommate so you can live in peace and quiet.