I think that there is a difference between accepting the other for who they are, and expecting the other to change because of who YOU are.
I don't expect my mono partner to have other lovers, because she is mono. She doesn't expect me to restrict myself only to one, because I am poly. But we can both accept that the other has a different basis for thinking about loving relationships that the other.
It's a choice what I do with my life, yes. I cannot choose who I fall in love with, though. Monogamous folks tend to only fall in love with one person at a time and, once in love, tend to naturally exclude those feelings from others. Poly people can't do that anywhere near as easily - it has to be forced. Can it be done? Yes of course! Does it feel right to do it? Quite often, not.
"Listen, or your tongue will make you deaf." - Native American Proverb