From what I've read of your other posts, I think I can understand why seeing that look on your husband's face was such a trigger for you: it's not your insecurities, it's because deep down you didn't want to believe that your husband truly loves such an untrustworthy woman.
She's clearly got some issues--is she still cheating on her boyfriend (with your husband)?? Ick.
For what it's worth, I think your husband is being a bit immature in his choices. I also think that a year from now, it's unlikely that he and his girlfriend will still be together.
Also, I wanted to point this out because it occurred to me when I read some of your earlier posts: you and your husband have different ideas about what kind of poly relationships each of you wants. You have "lovers"--men in other cities whom you can visit for caring, sexy friendships. He has a "girlfriend"-- someone he wants to be very integrated into his life, to meet all the friends he shares with you, to go out to dates/events in a public community that overlaps with yours.
Your lovers don't overlap with your husband's social life, right? So that means your husband has it easier than you, kind of? Because you have to struggle with having your own social life impacted by his girlfriend (his emotionally immature girlfriend). But he doesn't have to deal with seeing you with your lover(s) at his social events.
My opinion on your earlier posts was that your husband was being a bit immature and selfish to want a girlfriend who is so integrated in your lives, and then to choose a girlfriend with questionable issues. But I'm biased because my personal preference is for lover-type relationships, each independent of each other, with less integration than relationships with the boyfriend/girlfriend label.
I like the idea of two married people being able to date as individuals and to have other lovers. I know your approach is to try to step back and stay out of your husband's relationship with his girlfriend and let him conduct it how he chooses without a lot of boundaries from you.
But that would work better if she was more like your lovers.
It seems like if your husband's girlfriend was someone he went off to visit for a weekend or an evening every so often, but otherwise you didn't have to interact with her, then it would be fine for him to date a woman with cheating tendencies if that's what he chooses.
But if he wants her totally integrated into the social life shared by the two of you--then no, it's not really fair for him to ignore your bad feelings about her.
You sound intelligent and rational and you are clearly able to do the hard work of dealing with your own insecurities. But I don't think that's what you're dealing with here. I think your husband is besotted and he's asking way too much of you.
Single, straight, female, solo, non-monogamous.