Why did she stay with him?

Because she loved him. I'm not saying that's a good reason; I'm saying that is the reason. He loved her too. Yes, he had an affair, and I'm certain it was only one, but I do think he loved her. I think infidelity is one of the worst crimes one can commit. If you love someone and you commit to them then why would you do such a thing to them? And so while I do not think he should be absolved of his crime I do think he truly loved her, and that she truly loved him so that they were able to work it out and remain together. I, myself, am unsure if I could ever remain with someone who promised me their all and then gave it away to another. I could forgive them if they fell for another and honestly came to the conclusion that their happiness lies with the other person and so they must leave me. But I could not do with the lie and the wanting their cake and eating it too. But I've never been so much in love as these two characters were so I cannot say what I would do if the situation arose as it did for them. All I know is she loved him enough that she wanted to stay with him despite what he had done.