I will agree that being condescending isn’t cool and that no one relationship style is inherently better than any other. You are also correct that no emotion is strictly speaking healthy or unhealthy but jealousy is largely about insecurity. The only reason to mind if your beloved is sharing intimacy with someone who is not you if you’ve previously agreed to that is fear-based. Do you get jealous when one of your friends or family members spends time with someone who isn’t you? Why does being able to care about more than one person seem unlikely? Love actually is an infinite resource (although time is not) and having more actual love in the world is also a huge positive as far as I can see.
Even poly people experience jealousy from time to time, and the remedy for that is to look to what you need to be giving yourself in that situation rather than how your partner needs to change for your comfort. It promotes each individual being responsible for their own emotions as well as having real honest communication about their wants, needs and boundaries. Yes, if agreed upon boundaries are being violated, emotions like jealousy might be justified, but the whole point of poly life is not some kind of adult spring break free for all. It’s to co-create relationships that work for both/all of you before hand and then to engage in them honestly and with respect. If that is taking place, jealousy is a maladaptive emotion that the person having it needs to address.
My husband and I opened up our relationship several years ago and did go through some transition pains, but it was very honestly dealing with insecurity that made the jealousy pretty much go away. We only see other people together, even if I’m with my male partner whom he is not intimate with. James doesn’t feel jealousy seeing me with the other man I love but he would have a harder time being left at home and feeling excluded — so that’s how we deal with that. I find it really hot to watch him with another woman (to be clear we don’t just watch, but all are involved when we are with someone else). We’ve found that nothing else puts us in the present as much as having a third partner; it breaks us out of our ingrained couple rolls and has brought a lot of growth and happiness to our lives, as well as fun and adventure.
Poly life isn’t for everyone but compersion is a real thing. We are much healthier, happier and emotionally mature people since we opened up. It doesn’t mean it would be that way for everyone but I think you are trying to overlay monogamy paradigms over something that is completely different in just about every conceivable way and not surprisingly it doesn’t fit.