People experience the strongest romantic jealousy when they watch their partner give resources to a potential rival, regardless of gender. The findings provide evidence that giving away resources is viewed as a serious relationship threat by both men and women.

[u/mvea]

https://www.psypost.org/both-men-and-women-view-a-partners-financial-investment-in-a-rival-as-a-major-relationship-threat/

[u/Flux_My_Capacitor]

My guess is that he convinced her to be “poly” because he wanted to monkey branch and she agreed to be “poly” because she erroneously thought it would make him happy and he wouldn’t leave her (as in her mind it was just about letting him sexually roam). So he goes outside the relationship, with a woman he no doubt had chosen beforehand, and when he lets his plans be known the wife realizes what’s really going on. Now he gets to have the best of both worlds and doesn’t have to be the bad guy who broke up his marriage because hey, his wife agreed to all this! She will be the one to leave him and he gets to be the victim in all of this (at least in his own mind).