I will give you another example from my perspective.
For me a loving relationship is accepting the person for who they are. Not who I want them to be, but who they truly are. There are all sorts of articles written about self-knowledge and self-discovery and I fully believe that every human should have the opportunity to learn whatever they want about themselves. Therefore if I love someone I want them to have that freedom, and not dictate to them who they should be in order to be "good enough for me".
So, growing out of that paradigm, if my partner suddenly comes out and says they are poly and want to have "permission" to fall in love with multiple people, the choice is mine as whether this is still a basis for a relationship or not. It doesn't necessarily mean that I have to try to fall in love with multiple people, if that is not what I want, no?
There are some people who feel that because they are poly their partner has to be too - I don't feel that this is the best way to approach this, and I kind of question the motivation to do this, because it says "this is what's right for me, and I need that same to be right for you too".
There are quite a few examples or working mono/poly arrangements - the most famous around here is obviously redpepper and mono, although mono came in to an already-existing relationship. I have one where we were monogamous and I became poly, with my mono partner remaining very monogamous but accepting.
In summary and to your point - I don't think that either partner expecting the other to change for them is a healthy thing. Poly people expecting their mono partners to become poly or mono partners expecting their poly partner to become/stay mono.
"Listen, or your tongue will make you deaf." - Native American Proverb