Tell her to ring you up when she doesn't have a boyfriend. You would like to be with her as her boyfriend and if she's not willing to do that then its best to just get on with your lives without one another.
Why let her make you her emotional tampon? Why end up in a sexual relationship with someone who has a boyfriend? Why let her make you her male GIRLfriend? Why let her demote you from lover/friend to just friend?
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion