I would react the same way if I were you. You and your partner set boundaries and he did not respect that. He probably didn't do it intentionally, but he wasn't mindful of his actions and his relationship/feelings went further than you were comfortable with. However, the fact that he let this woman believe that ya'll were poly concerns me. It seems that he lied (even if it was an act of omission) in order to maintain a relationship with the both of you; maybe to spare both of your feelings or make him seem more ethical in the eyes of his new lover. Either way, that’s not okay. I may be new to this, but it’s my understanding that poly relationships only work if all parties are honest and open with one another. That means setting boundaries, but being able to communicate your needs/re-negotiate the boundaries if they are hard to maintain; not lying to your partners when you are having trouble maintaining a certain boundary.
So if you decide to stay in a poly relationship with this man, he needs a talking to about respecting boundaries and having the courage to communicate his needs (instead of going about it in a dishonest way). You can always tell him that if he doesn't get his head out of his ass and start handling himself in an honest and respectful manner, that soon he’ll have zero girlfriends