Well, yes, EM. I think that's what usually happens - deliberate cheating must be the rarer event, surely? They were friends, they grew closer ... they nearly had sex, they called it off ... they missed each other badly, they had A Conversation, they set themselves a deadline (6 weeks, iirc) ... they made their minds up, he told his wife.
He purposefully cheated for those six weeks and I have to say I see why he did. He was facing a painful decision. To go home and ask his wife to discuss would simply have brought the decision forward (indeed, would have forced it.) There was little to discuss; he was not particularly unhappy with W#1, but had discovered new depths of feeling and was trying to interrogate himself.
I know one other couple who were married when they met. She was already in the process of leaving her abusive H, he was bumbling along in a mutually-critical marriage. These two, also, ended their affair when it threatened to become physical. They stayed apart for a year, then spent 2 weeks together.
It's easy for me to say "I don't do married" but I haven't been in the OW situation - my close male friends have always been just close friends. With the first couple, where I knew all 3 participants well, I couldn't find it in myself to call any of them wrong. I shared his conviction that, had he met W#2 first, he wouldn't have married the first time.