It is definitely a mono mindset to think that a partner who wants poly will choose one person over the other. One of the points of being poly is to have both... or more... so it takes tossing away your old frames of reference in order to wrap your brain around the fact that most truly poly peeps aren't gonna go anywhere. Having more than one relationship means having more than one -- it's not about auditioning people to be the top pick and leaving the other(s).
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein