You feel betrayed...who wouldn't?!
Announcing being poly after cheating is not cool. No, I don't think you are being too harsh. What happened and the subsequent revelations has got to be just awful for you.
I don't see how they can think you are over-reacting if they have an ounce of compassion or even self-honesty. I call "bullshit".
This has nada to do with you being okay being a mono in a relationship with a poly and everything to do with being betrayed. Don't torture yourself over that particular issue, ever.