I never liked my husband's one room mate and dreaded time we spent with him. 37 years later I still have to see him now and then and I hate it. I totally understand how you feel.

Why don't you tell your BF that you want to spend more time with him alone and ask if you can do things just as a couple? You can also tell him that you find his room mates comments offensive and really don't want to spend time with him. It sounds like you did not invite him to join you. He is just there since you are at his place. Go somewhere else. Do something else. If he is good friends with your BF, give them time to do things together without you. With luck the room mate will find a girl and spend time with her and not you.