Thanks for the update Nyjah. I assume you've tried saying no and she just overwhelms you either with force or seduction. It may be that you need to find a new place to live, away from this roommate and his nymphomaniac girlfriend.
A guy who enjoys seeing his girlfriend hop on his roommate against his will, is not much of a friend. A girl who gets off on forcing guys into sex is not much of a girlfriend. I say ditch these two as soon as you can.
"I get up, and nothing gets me down.
You got it tough. I've seen the toughest around.
And I know, baby, just how you feel.
You've got to roll with the punches to get to what's real"Van Halen