Just tell him that when you're in an exclusive relationship with a guy, you expect him to stop hanging out with other women and that because he's not doing that, then you're unable to continue in a relationship with him. Wish him luck in his adventures and then with head held high.. walk away from him. He's not ready to be in a relationship and be loyal to anyone... not just you.
There is lots wrong with having female friends if you continue to hang out one-on-one with them, keep them separate from the primary partner and are sketchy in your behaviour with them. It's very disrespectful to hang out one on one with your opposite sex friend, its particularly disrespectful and inappropriate if you've never met her.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion