Once A Cheater...

I really truly wanted to believe that it was an honest mistake. My lovely boyfriend hasn't changed and I really don't believe that he's going to. He sends dirty pictures to whichever woman he happens to be talking to at the moment, he receives and keeps all the dirty pictures that the women are willing to send. He writes XXX short stories for whomever asks, and then has the nerve to be upset with me for talking to another guy. I am with him monogamously, I don't send or receive anything dirty unless it's from him, but he sees fit to send and receive everything under the sun just because he can and "that's who he is." I hate that he does it and I've told him that, he said he would stop and he wouldn't do that anymore. I don't know what to do...