Re (from Messenger, Post #19):
"I was not aware of him being in a poly relationship with a girlfriend until after we had already been together. I assumed when he said, 'You should meet X,' that he was talking about a friend or FWB."
Yikes, he wasn't even being 100% upfront with you at first! (If he was, he would have said, "X is my girlfriend; I'm in a long-term relationship with her.") I mean, for him to wait on being 100% clear toward you until after he had been with you ... [skin crawling]
That said, it's good that you're aware of her (potential) feelings, and have backed off until he tells her. Definitely get personal confirmation from her if you can.