What I keep getting out of the OP's posts is the sense of revenge as well. She hurt him by wanting another love in her life despite still wanting him. That request made her capable of anything in his mind including cheating before asking to become poly.
Instead of saying no or choosing to leave the marriage for the disparity of values he has stayed till he found someone to move on with. Ready with the unprovable suspicions of cheating to absolve motivations. This allows him to "do it back to her" before leaving. So not poly like her in any sense other than throwing the wife the occasional bone. His mind is still mono. The GF as the ready replacement when he goes. But he has to make sure the wife feels as he did, replaced, before he goes and has to make sure both his wife and her BF know he is replacing her.
Of course I'm guessing here, but I doubt this is what his wife was asking to happen in being poly. So that suggestion of this being the result of what his wife asked for doesn't seem to hold water. This isn't the natural result of anything. If this wasn't the result he wants to have happen and simply how it all played out, why the request that the wife become the secondary to the GF? Seems a bit more stringent that just wanting to do away with the labels.