Very good piece! You’ve hit on so many salient points here. I live with James but am also partnered with Nat. They know each other and have spent time together, although don’t talk much independent of me. Still, I pass along greetings or questions from time to time, and on rare occasion they have communicated directly.
Nat has a partner whom I have never met or talked to. Like you, I have grown to care about her deeply. When she recently went through a health crisis I felt very protective of both her and Nat. I adore their children (whom I have also never met). I would much prefer that she and I have a relationship more like the one that James and Nat have. And, I also respect that one of the upsides of poly is that you get to navigate relationships in the way that works for you. Some people need compartmentalization to deal with a way of being in relationship that is largely not supported in our society. I don’t like it, but I do understand it, and allow for it (as if I had a choice 😉).