Thanks, Anneintherain. I haven't seen anyone else discuss a comparable situation in a forum, ever, where a partner wanted multiple sexual relationships despite a low libido. It's reassuring that I'm not alone.
I may be quoting this to him: "I remain convinced nobody ever died from not having sex with somebody they wanted to have sex with."
I'm not ruling out him getting more serious with R in the future, but right now, I don't feel good about that. We are currently having sex about half as frequently as we normally do, and it's hard on me. It almost feels to me that he gets unconsciously confused when he has more than one sex partner and his libido shuts down with me. I don't know what to do about that. Sexually, we always seem to do better as a couple when he's not in any other relationships.