It sounds like your bf's roommate has a problem with commitment with one girl and only one. Your bf is best friends with him so he get defensive and doesn't want to look at the facts. You need to stay away from the roommate and probably ignore him and just go straight to your bf. Keeping on the defense yourself with the roommate is important protecting yourself. Is seems like the roommate likes you. Just becareful. If you showed him the facts of his roommate and your bf doesn't believe you and defends him, then I probably would get out of that situation.