To add to the poly/mono experiences, I am poly (but with no other relationship at the moment) and my wife is primarily mono. My wife doesn't desire other relationships or seem to be filled by other relationship like I know I am. She likes alot of space to herself and is quite happy to have our relationship a mono one.
I am not happy with this and have come to recognize that this is not who I am. My wife doesn't want me to be unhappy so we have embarked on a long and painful journey to come to some resolution.
If I start experiencing other relationships, my wife doesn't want to miss out on the "fun" so to speak so she feels like she really would like something else as well - probably more casual in nature like a FWB whereas I would like something deeper and more connected.
So in some ways our experience adds some weight to the thought that it is very hard for a strictly mono person to be with a poly person. Even though my wife definitely leans more towards the mono side, if she is going to accept me being and practicing poly, she feels she needs to be in on the action at least to some degree too...