Maybe the partners just have respect for how much their partner has had to put up with for their sake, rather than being indignant and defensive.
I think there's a difference between active contempt for a person, felt most of the time VS feeling tired, stressed, upset etc. some of the time, due to a particularly tough period or scenario. The latter feels more worthy of respect than the former in my opinion. A person doesn't always have to react defensively or with indignance, maybe they are just sad, concerned, worried, surprised, disappointed etc.
I wouldn't want to be friends with someone who had a habit of feeling contempt and showing negativity towards any of my family members. Let alone sharing a life with a partner who feels this way about my kids. I would think: move out if it is that bad and that hard. Maybe i might get into the relationship because I didn't know it would be this way, but then i would breakup because it is so awful for everyone involved? It is hard enough when partners feel negativity and contempt towards in-laws or friends.
Obviously everyone is different, but I wonder what is it in for both adults. Maybe the rest of the relationship is wonderful so it makes up for it, but 50% of the time is a lot of time to feel resentful. Or maybe the SC are adults, or they could be children but aren't around that much.
I just find it difficult to really imagine what the different people are feeling in the moment, why they stick around, how they are coping. It all sounds very toxic, but maybe it isn't in practice. Each to their own. I don't have to understand, I guess.
@aSofaNearYou Although I don't fully understand your POV and experience, thank you for taking the time to reply to my previous posts.