Would you be upset?

My boy friend was I voted to a pamper party by one of his long time female friends (he slept with her at least once a long time ago, and says he doesn't now). She has obviously stuck her leg up between his whalie me and him have gone out. He never invited me but kept telling me how he didnt want to go. After he went I asked a lot of questions because I am still not at ease do to the previous situation. He mentioned everyone except for a girl that saw in a picture from that night standing up close to him, with her hand on his shoulder and her knee bent on his leg. He claims they are old friends and is very angry at me for even being upset. He said she is married and I am being jealous.