It took time for him to get there, but ultimately it was him dealing with his insecurities (which stemmed from childhood issues and didn’t really have anything to do with me or the current situation). Jealousy is always about insecurity but societal programming about what marriage is supposed to look like didn’t help matters. It’s hard to imagine things we’ve been told are aberrant and destructive as actually having potential to be good and beneficial.
Reading more about ethical non-monogamy and being exposed to more people who subscribe to that helped. Then he also started to see how much fun we could have, how much it actually expanded our relationship and didn’t really harm it. We had deep and meaningful conversations about things we’d never told each other before, even though we thought we’d had a close relationship before. We both grew as individuals and our relationship grew too. It was a process, but one that was well worth it. We’re both very happy now, with each other and with the life we lead.