What if I don't immediately leave the room when I walk in on my roommate having sex with his girlfriend?
Are you joking around? Why would you stay in the room during that?
I would feel uncomfortable if someone walked in on me having sex, but I'm not sure if my roommate and his girlfriend feel the same way.
If you don't want your roommate watching you have sex, you shouldn't stay in the room when he is having sex either. It's common courtesy to leave if you walk in on that. Your roommate might be very upset with you if you do that.
Nah, I can do what I want.
I don't think that's right, though. Your roommate might want to stop sharing an apartment with you if you do inappropriate things like walk in on him having sex and staying.