It’s an odd phenomenon alright. South Park covered it in the episode “Sexual Healing” where famous rich men kept cheating on their wives. We may never get to the bottom of it.
Marriage/long term coupling is not "natural" but normal in society
With the decline in religious attendance has come a decline in marriage; from 70%+ to 50%ish over the last couple generations
But divorce rate has declined so those that do marry seem to really want it; in the before times though it was how women survived
Coupling fits with biological imperative to have partners but longterm/for life is obviously not necessary since divorce is common too, partners die, etc
That and recent research shows long term married couples are 50% more likely to develop dementia; an imbalance in effort develops where one becomes codependent and forgets how to live and the partner in charge loses grip of themselves and taking care of their partner consumes them.
it's also possible the wife knew and was okay with it
my wife and I date other people, you'd be surprised how big the ENM/Poly community is when you start looking