It's not always about the commitment to the marriage but also being a decent person. A decent, moral person would not think that's ok.
Also in my case, one was my brother in laws long time, live in girlfriend at the time. She was known as aunty to the kids. So, yes, she is at fault equally for the break up of my family, brother in laws family and the family dynamic changes in general at that time for her part in it when she knew full details of our family.
The cheater is still very much at fault. But to say the other party, if they knew the situation completely, is not at fault at all is not how I see it nor will I ever. So I get what your saying, I just don't agree one bit. My anger will never go away for either party.
And yes, their life failures as they happen will always bring me great joy. I hope they continue to experience misery and heartache as long as they live. I will never feel bad for it. Maybe if they had been good people projecting good karma they wouldn't be in the situation they are now.
I do feel sorry for the kids though, being brought up by someone with low morals, not caring for anyone but themselves, is not a good environment and leaves them vulnerable to learning the same traits.
Yes I'm still bitter about it all, I probably always will be to some extent. I don't stew on it anymore, nor think about it much but when if I hear their lives have gone downhill it will always bring me some happiness in that they have brought any bad karma on themselves and they deserve every bit.