Okay, I'm just a vanilla mono, but as I understand it, your husband told you he wanted to try poly. When you didn't embrace the idea enthusiastically, he moved forward to telling you he's now in a poly relationship and it'll all be great, even as you're telling him it's not great, for you. And the man of the other couple has strong feelings for your husband??
Are you interested in being part of a poly relationship in general? Is the problem poly or this particular couple?
Because to me, this doesn't sound like poly, which as I understand it from reading on these boards is about honesty, mutual consent, love, and respect. It sounds like a guy informing his wife he's now seeing someone and she can like it or lump it. There's a little bit of honesty there, but no mutual consent, no love and no respect.