It sounds like you're a good lady and loving wife, JLynn but emotional/internal conflicts make the situation difficult .... though maybe you feel threatened in your relationship with your husband by the other woman for some legitimate reason(s)?
I took my wife to meet a lady I'd met on-line and knew for quite a while. My main concern was that she and my wife got along and that my wife didn't feel too uncomfortable or left out. It was primarily just a platonic/friendly visit to see how everyone got along and it went fine, though I know the relationship may not have been ideal they really did get along surprisingly well and I was happy to see it
Things didn't ultimately work out with her, but for me, I became even closer to my wife and appreciated her more because, similar to your comments, I realized how much he cared about me.
I'm just curious if there was some reason you might have felt threatened by this other woman. Yes, I agree that in many ways a poly relationship can be better all around if everyone gets along well, but of course if there's some conflict then that's something that needs to be resolved, but I'm curious to know if you think it's just a matter of it being difficult for you to "share" your husband (though hopefully gain a companionship with her as well) or if you feel legitimately threatened in your marriage by her. It would seem if you and your husband have a good relationship that she shouldn't be a threat to that and that if you're not inherently opposed to having someone else in the relationship then there would be no reason for him to leave. The main issue would just appear to be if there was something specific about the other woman that caused a conflict.
As a disclaimer, I have to say that I haven't had any serious long term relationship with anyone other than my wife, though hopefully I've helped give you something that might resemble your husband's perspective some.
For me and my wife the main things I can think of that might cause a problem for our marriage would be 1) if somehow I bumped into some absoutely incredible woman who seriously tried to pull me away from my wife (this is something that seems unlikely ... I already did a lot of "shopping" finding my wife and we have, other than some rough periods, a very good relationship, 2) if a long term relationship with more commitments developed that initially went without major problems but later some conflict between her and my wife occurred, then it would be harder to resolve or 3) if my wife simply opposed any and all female company for me (that would be a tough one and I think in the long run would cause ongoing conflict in our marriage).
Anyway, I hope my two cents worth was actually worth two cents.
I also wish you well, and again it seems like the major issue is in knowing what specifically causes the conflict for you (is it her specifically or just any woman) and seeing if you can work with your husband to resolve it.