Well, I tend to agree that the mindsets are very very different, but it's human nature of compare new experiences with our existing paradigms. Folks need to have information and try to fit it into their "world picture".
A lot of poly folks struggle to understand mono folks - so often they will post "why don't they get it? This is just so much commonsense!" The thing is, this mirrors exactly what the monogamous person is thinking.
Based on what I have seen and experienced, it's worth exploring a little what common ground there can be - it might be that the mono person is more poly than they think, due to conditioning, but there comes a point where you have to stop beating the issue to death and start to take a few things on faith about how the other person thinks. If you wish to stay with that mono person, then there may need to be some extra concessions put in place in order to keep things from going totally off the rails. It's going to differ, based on the individuals involved, of course.
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