no one is stopping you from being poly

While that's technically true, it's a lot more nuanced than that.

Our society instills monogamous values in its media and traditions. The sentiment around polyamory is that it's weird, naive, frivolous and immature at best. The very suggestion of polyamory outside of friendly circles is often met with negative vitriol, as evidenced by the amount of down votes I'm getting in this thread.

I recognize now that I have always leaned towards polyamory and that's been an awakening that's taken nearly a decade over the last third of my life. I never cheated but I went through deep emotional pain and have caused emotional pain to others trying to figure that out in a world that was telling me monogamy was what I needed to strive for if I wanted a meaningful relationship. Even now, I'm voluntarily in a monogamous relationship because I deeply value my partner, though I can't help but wish society hadn't scared me into rejecting that part of myself for the better part of my life.

The problem in this case is when he wants to see other people and his wife believes they're in a monogamous relationship.

I never argued against that. Cheating in any relationship is deceptive and immoral. My qualms are with the fact that our society is biased against non-monogamy and due to that many people don't realize it's an option and instead resort to ways of getting their needs met that cause emotional harm and turmoil.