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.
Compulsory heterosexuality has been the standard of our society for centuries up until very recently. It was common for closeted gay men to marry women in order to fit into society and then have secret affairs with other men on the side to satisfy their homosexuality. That's changed over the last few decades but even now it still happens. Would you have told a gay man from 50 years ago that compulsory heterosexuality is nonsense? That nobody is forcing them to fall in love with and marry a woman? Despite the fact that every social normality and structure was oriented towards heterosexuality and fiercely rejected homosexuality to the point where homosexual people closeted themselves out of fear for their own lives.
I'm not saying non-monogamous people have it as bad as homosexual people did in the previous era. But you can't deny that our society strongly encourages monogamy and strongly discourages non-monogamy through social norms and structures. Similar to homosexuality in the previous era, there are little to no widely available resources for non-monogamous people to help them understand that part of themselves and the vast majority of them don't consider it to be an option due to the stigmatization of non-monogamous relationships; in other words, compulsory.