I feel really sorry for the girl. She's obviously not been taught any self-respect or has much if not any parental supervision in her life. Her mother needs to be held at fault for even allowing her child to behave this way. This isn't just a matter of a girl being slutty. This girl is damaged emotionally and is continuing to damage herself.
She needs some kind of counselor or mentor in her life. A boyfriend doesn't count. Max I am actually not going to advise you to dump her outright as some of the others in this thread, but is there a way that you can help get her to a counselor of some sort?
I would say that you should talk to her about seeing a counselor before the two of you continue your relationship down the same path. Perhaps offer to be her friend until she gets her issues straightened out?
“Inside every cynical person, there is a disappointed idealist”--George Carlin