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 went back to the doctor
To get another shrink.
I sit and tell him about my weekend,
But he never betrays what he thinks.
Can you see the real me, doctor?. The Who