I think he is one of those guys who get in love with you because you butt looks nice. Hey, my opinion is that he didn't used you, you got to understand that he can't live without sex. Telling him that you can't do it with him no more, broked his heart in another different way. You know, i think it was better for you to leave him, why? well look whats happening now, he is going out with your best friend, she wasn't a best friend also, he wasn't a good boyfriend also too, they both didn't respect you. I'm really sorry that happened to you, i know you still love him alot, that why girls have some reasons to say that we guys don't love ya as much as you love us. i know theirs better guys, but hard to find. But i know theirs some one for you, try to know the guy first, and if he turns out like your ex, try to change him, making somebody change is nothing against love. If you love someone, just try to be the best boyfriend ever.
Do me a favor, Don't hurt yourself, i hate seeing and knowing girls like you are suffering for a guy who is not even worth it. I mean, this is what i think, probably some other people qoute different, i seen relationships like this, and the girls are always to get deppressed and hurt while the guy is just having fun with another girls...
Take care of yourself, theirs someone out there waiting for you, you will know