I think that you should not be the "other chick." You're just going to end up degrading yourself. Where is your self-esteem and dignity? You're throwing it all away for someone that probably just wants to grow his male ego. What about his gf? Doesn't it make you feel bad that he wants to be unfaithful to her? What if he broke up with his gf and decided to come back to you? He's just showing you that when you guys were together, he most likely cheated on you! Don't fall for his games and move on.