Is my roommate/friend right about this?

My friend/roommate and this guy hook up every now and then but they aren't really dating. She also hooks up with other guys as well. But however I like this guy a lot and I think he likes me so I've been with him behind her back and we have gone out a few times so it's not just sex with us like it is with her but anyway she found out about it and got mad and said I stole her guy! But he's NOT really her boyfriend though so I don't get why she's so mad. Plus she said I have to move out at the end of the month since she didn't want to live with a backstabbing s!ut! but I don't think that's fair at all. Am I right?