Where did you read about the Matthew Fox part of the ending? I read about the Moscow scenes but not about his wife having an affair with Matthew Fox's character.
There are a few articles, I think there was a big huffington post one which went into details and they discussed it in an IGN video. It's been in most of the articles I've read about the original ending. I don't think it was technically an affair either, she was sleeping with him so he would protect her. I can understand why they needed to change that part of the ending it sounds terrible.