So are your fears truly about being left, or about feeling a level of jealousy and hurt that you don't think you can live with?

Both, of course. The jealousy and hurt is, in part, a rejection to a fear of being left or rejected. And in part it's just base possessiveness, probably. I'm good with that.

you can find the source of these feelings

I'm answering questions about these feelings because I'm in a discussion about them with people who presumable don't feel them (or not in the same way). I'm not confused or in the dark about the source of my feelings on the matter. This is not the first time I've thought about my feelings, just as I'm sure when you explain why you're okay with poly, it's not your first time working through these thoughts either.

You choose (and are allowed to change) your deal-breakers.

Sure. But why would I, when I have zero desire to?