Just tell her she can't have it both ways...that you aren't comfortable with her still being with her girlfriend..its basically cheating..on you..and on the girlfriend....she's obviously Bi-sexual..which I personally don't believe in the term...so you always will have that in the back of your mind..."Is she dabbling with chics?" while she's with you...she doesn't have a problem cheating so...take that into consideration also. Good luck though.