I kinda have to disagree, she pretty much immediately claimed ownership over a child that was in no way attached to her. It was also made known later that Carol in fact did cheat with Susan. Carol is to blame too, but the way Susan treats Ross even though she was in a relationship with a, then married person, is just straight up horrible. Not even just the baby name thing, but immediately hogging ownership of the unborn child and actively trying to push Ross away from his own child was just horrible.
I do agree that when someone cheats, the blame is usually being put to the unmarried/single person in the dynamic which is just straight unfair, but the way they both kinda treated Ross was unfair too