Great comments on this thread. I like SchrodingersCat's view especially.
However, there seems to be an assumption about protecting the primary couple here. The trashtalker is the one who is outside the primary couple...if he/she is trashtalking, he/she must be crazy/immature/not worth anyone's time, etc.
The flipside of that is this: if you are dating someone, and you think their spouse is selfish and manipulative (and maybe you have rational, reality-based evidence for that view), you need to get out of that relationship. It might be tempting to badmouth the spouse...it might be tempting to use misogynistic and hateful words like "bitch"...but really, you need to dump the person you're dating.
Why would you want to be with someone whose spouse is so awful? Someone who is devoted to a selfish and manipulative spouse must have their own mountain of issues.
Single, straight, female, solo, non-monogamous.