I have a husband and boyfriend much like the two of yours. The NRE I had with my boyfriend Mono was similar. I found it hard to believe that he wouldn't leave me for a mono woman. He might well do that one day, but I doubt it now and have since decided to not waste my time worrying about it.
I think that given time the NRE will wear off and you will trust more and be more relaxed about it. He says he wants you and you only and laughs at your insecurities around this, so try and keep it light and realize he is just being himself and enjoying his flirting and fun with the ladies. That would be my advice. Its harmless fun and perfectly normal after all. At least in my opinion.
Perhaps joking around on his fb would help. It did for me. I teased him about it on there and so did the women he flirted with. Now I don't even look because its harmless and I'm not worried or interested. It would be a shame if jealousy became serious and damaged your relationship.
There is a lot on here about jealousy if you do a tag search and a lot of stories that might help. Perhaps checking out some of Mono and my stuff would. We are all moving in together now, so perhaps earlier stuff when we we were full of NRE and angst about poly would be more helpful.
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