While it is apparent that most of us have serious reservations about allowing another relationship to dictate the terms of our own, Nycindie brings up an interesting psychological point. When another has limits about time spent with us, are we more forgiving if it is a job issue, a child issue, etc limiting the time than if the limit is imposed by an insecure partner? If the answer is yes, then why?
I know for me, and from past experience, it is the fear that the limit is a slippery slope. One limit became more limits until she forbade the relationship entirely. So the question becomes, is the limit the other partner asked for unreasonable? Or is my reaction, not to the original request, but fear about how such requests may morph?
That said, I have known monogamous couples who were jealous of their spouse's career; I have known people dating a divorced person with children who were jealous of the children. (I mean really?)
Poly people, out of necessity, become very good at understanding their own motivations.