This just really pisses me off.
Poly isn't "cheating". It's not........oh guess what honey, I've fallen in love with someone else and fucked her, without you knowing any of it.
I'm so sorry that you are going through this kind of thing right now.
I guess the real question at hand is are you okay with this? Do you think you can restore your marriage even after what has happened?
On the defensive........your man probably didn't feel he had any other option as society teachs us that loving more than one person is bad and you can't do it. If you do, you must hide it.
But he should have talked to you about his feelings for her before it went sexual at all. Once it crosses that line, IMO, it's not poly.....it's cheating.
However, my opionion doesn't matter at all. What matters is how you feel about this.
Can you forgive him? Can you walk the poly life with him, and her?