Hey, I would be mad too. Just like someone else said, it is emotional cheating. He said that he was insecure and needed the attention. Well, it seems like he was the one seeking out and giving the attention. It's not the same as someone flirting w/him and him feeling good about it.
And if that is the excuse he used, what did he become secure all of a sudden. Also, what else has he done that you haven't found out about yet? Not saying that is the case, but.......