Okay, the situation isn't ideal, but to be honest I think you should respect her wishes here. Having a stranger in your dorm is an uncomfortable thing - I share a room at uni and adore my roommate, but when she has male friends round I do find that I can't behave in the room as I normally would, and it's harder to relax. Best thing to do is to talk to her about it a bit more and find out why it makes her uncomfortable, but when you're sharing the place you sleep (which is pretty intimate, really) with others you have to be willing to compromise and consider their feelings.