Glad you could join us.
I don't think there is any quick way of recovering from a breach of trust. In a way, things may have changed permanently, as you now know your partner is not "perfect," and that he is capable of getting caught up in something he knows is wrong. But in time, if he continues to show evidence that he can be trusted again, you may slowly begin to feel more trust in him. What you have to decide is, what is the extent to which you can forgive him.
Yes, it's true that any goings-on conducted without the *knowledge* and consent of everyone connected to the relationship, is an affair, regardless of whether the persons involved identify as poly. It sounds like your partner is quite taken by this other woman, and I wonder if it won't be very difficult for him to let her go.
Try not to make an major decisions until you have had some counselling and some time to think about all this. More light needs to be shed about why this happened, how/if it can be prevented in the future, and what it means to you and your partner as a couple.
I hope you find some of the answers you are looking for on this site.
Love means never having to say, "Put down that meat cleaver!"