"Does he also love the woman he’s lived with all this time? Did he know her when she was younger and continued to love her as she grew older?"
(EpsilonLyr, thank you and I agree with you about the internet. I am very grateful for all the help; it's a huge boon. Internet means more human connections, and I think everyone here is on board with that!)
He's an extraordinarily loving human, very good to his lady. It's pretty domestic between them though; they take care of each other, six years softly negotiating the rough world together. If love is counted in patience and acceptance, I bet they score pretty highly. He says he's never stopped loving anyone, which I kind of understand. But I think love can morph, and sometimes it morphs into something unsustainable.
She's monogamous and I have a really hard time believing she's all that happy with the situation. I know that, unlike with other girlfriends he's had, she's not very comfortable with me, rather wants to keep this a--what do they call it?--V relationship. She and I are like night and day, Betty & Veronica sort of thing. Plus she's the face of my guilt, so I don't really want to go have a mani-pedi with her either.
I don't seem to be monogamous--still working it out. I'm not sure how open he is with her about us--hence the year-and-a-half-long restraint that makes me grind my teeth at night.
This is all just new to me and I can't put my finger on the line between permitted cheating and more than one true love. People fool themselves about their motivations sometimes. If he's just using polyamory to keep things spicy while she holds down the fort at home... well then he's the kind of man who won't hold my heart in good trust. See my emotional logic? Which is all silly because of the two of us, I'm the one more likely to betray or abandon. Working on that too.
The solution should be to talk things out with her, but I know her interest in that is low.