I agree with what has been said before me. Looking at polyamory or monogamy as the problem misses the real issue.
The challenge is finding a way of relating and loving that is compatible with our needs, desires, etc. Incompatibilities exist and relationships end. Mistakes are made and relationships end. Those incmpatibilities or mistakes may rub up against the "expectations and understandings" of a particular form. But, are the forms themselves "to blame?" These things happen irrespective of relationship form.
The real issues are deeper.
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”