You are sleeping around with multiple women - which is fine. You're sleeping with a girl who doesn't do relationships - ie she is sleeping around with multiple guys - which is fine, but what kind of girl is this? A girl who wants to sleep with lots of guys is not looking for a relationship. You're trying to change this girl and it's simply impossible. Accept her for who she is and what she wants.

If you want a girl who is relationship material, you need to re-think the types of girls that you date, and you also need to re-think your own behaviour.