OK,
So if someone doesn't want monogamy then don't sign up for it !
In todays society there are more types of non-monogamous relationships then ever before ; 'open' relationships, 'open' marriages, threesomes, polyamory, gay, lesbian, bi, BDSM non monogamy, DADT (don't ask, don't tell), soft swap swinging, - so plenty to choose from!
No-one is forced into a monogamous relationship.
What is totally unfair is when someone signs up to monogamy and let's the other party believe they are fine with that, and then cheats. That robs the other party of a choice in the matter.
If someone decides that they no longer want to be in a monogamous relationship, then the mature and sensible course of action is to tell the other party, to give them agency in the relationship.
Sneaking around behind someone's back with A.N.Other doesn't resolve anything.
Having an affair to solve relationship problems is like firebombing your house because the kitchen tap's leaking.