Welcome to the forums. I'm so sorry this happened to you. I've been cheated on (although it was before my poly days), but I imagine your pain and feelings of betrayal are every bit as severe as mine were. You never expect your soul-mate, the person who you depend upon to always be there for you and never hurt you, could stoop to such deceit. He knew he was doing wrong, else he would never have kept this from you.
I second everthing Schoedinger said. I demanded complete transparency from my W when this happened, in addition to no-contact, MC, and I even got in touch with the OM's fiancee to ensure this affair would come to an end. Does the "girlfriend"'s husband know about all this? If not, are you going to tell him? I've always had the belief that it's best to let him know, because if he were to find out through other means and know you were holding out on him, he would be very angry and resentful.
And I'm sure many here will agree with me that it's best to stop contact with this couple and concentrate on repairing that trust, which will take a commitment from your husband and lots of time.