Carol figured out that she was gay and she had every right to get out of the marriage and be with her own partner. But she did in fact cheat on Ross for a while and was with a person who insulted and antagonized the father of her kid. Carol leaving Ross for a woman was instead played for laughs. It kind of obscures the cheating doesn't it?

Ross was never a deadbeat parent and did everything he could as a great father. At the end of the day, she pretty much treated Ross like nothing more than a sperm donor was easily the worst thing she did. And then she had the nerve to call out Ross on sleeping with someone while being on "break" with Rachel.

Political correctness. It was a pretty progressive storyline for its time, so they weren’t going to position Carol as the villain, even though she would have been the villain if she were straight. It always bothered me how this was glossed over. As for Susan’s poor treatment of Ross: in reality, he would have made every attempt to keep his son away from her, including changing the custody agreement.