It seems to me a very cowardly way of him telling you exactly where you fit in his life. As in... You a secret, you'll never be accepted, get over it" I don't think your over reacting at all. That would hit me in the face like a wet towel.
Ultimately it's up to you weather you can continue on as you are.
I've been in a very similar situation, it was just easier for him to lie about me, not mention anything to his girlfriend, or her lovers. To much effort for him, I didn't give him an ultimatum. I'm not that person. Ultimately I decided that I wasn't going to be his secret "shame" anymore ... That's the way it made me feel anyway.
I hope you get the resolution you deserve *hugz*