My college boyfriend would accuse me of trash-talking or criticizing his primary partner every time I tried to talk to him about her. I would say something like, "Is she mad at me about something? She seemed jealous of me today and it felt awkward for me. Can you talk to her about it?" And my boyfriend would stare at me like I had murdered his best friend and tell me that his primary was never jealous of anything, she was wonderful and awesome and he would break up with me if I didn't like her. End of discussion.
Obviously, my boyfriend had major issues and my relationship with him was totally messed-up. This was way before I even knew anything about poly. Or how to stand up for myself. Or how to dump losers.
This might not be relevant to your issues, but it's what popped into my head.
The funny thing was, my friends and even my mother DID trash-talk my college boyfriend's primary. This upset me because I really liked her. I always defended her--even after her behavior did become manipulative and unhealthy.
I think there's a strong societal perception that any sort of "other woman" is someone who deserves trash-talking. She must be an evil, manipulative, controlling slut. This is obviously an attitude that should not be tolerated. But, it's so deeply rooted that good people whom you care about might spout this attitude on occasion. Point out to them how wrong it is.
However, in your case it seems like the real issue is, Why does your husband choose women who trash-talk you?
Single, straight, female, solo, non-monogamous.