Yes, like some of the threads on the forum, I was blind sided and felt utterly betrayed. It hurt for some time but not as much as losing the family I wanted.
I did not get custody of the boys. He and his ex-wife were constantly using the children as pawns in their on-going custody battles and when he met this other woman and I got the ultimatum, I offered to take custody of both boys provided both parents would provide child support. I had always wanted a family and these two boys were wonderful. In the end he and his ex-wife would not sign over custody. His new girlfriend sent him a ticket to fly to her - he gave up custody to the ex-wife who I believe gave them up to children's aid. (She had threatended to do so for a long time) I am not certain of the outcome as I was not allowed any communication with the boys once I moved out. Very sad.
It has been a long and careful walk back to the topic of polyamory, my own readings, joining some meetups and forums like this have been helpful and supportive.