Are you going along with this in the hope that it will "jump start" your sex life, or "enhance" your marriage?
I am always skeptical when a husband "realizes" he's poly shortly after the birth of the first child. I think i have noticed that some men are turned off by their wives' bodies once the wife becomes a mother. There are various possible explanations for why this could be. Not assuming anything about this particular case, just mentioning that this does happen and it might be worth thinking about, if not the OP, then someone else who might be reading this thread.