Thanks JaneQ, for your objective and insightful perspective. Your analysis is pretty spot on I think, and much less ranty than my original post. He did say during a big discussion/argument a few weeks ago that he "wants to be excited again," which certainly corroborates your suggestion of romancing him.
While I will definitely invest my time into that, I am concerned that the complacency of our relationship is only the tip of the iceberg. In our initial nonmonogamous arrangement, I was comfortable having exclusively sexual encounters with others while he insisted on a more emotional connection with his. I still don't understand it, but I do think he is more poly than myself. I'm fairly certain I can thrive in an emotionally monogamous relationship with sexual outsiders. I don't think I can handle emotional polyamory, but he may be wired for that, and so the purely physical nonmonogamy may not be a long term solution for him. I'll admit I'm being selfish. I don't want him romantically connected with this other person, or anyone else for that matter, and I don't feel bad about that. Furthermore, his venturing outside our connection for solace and romance felt a bit like a stab in the back. And I believe I can move past that of course, but it makes me feel disconnected and less-inclined to fight for him. Does that make sense?