You have a somewhat cynical view of things, and I do not agree in the slightest with your viewpoint, but hey it's a free country (depending on where you actually are :P). Some people choose to take a difficult situation and do the RIGHT thing, like a) mend it, or b) leave. Not everyone thinks cheating is the answer.
Personally, I've been in a situation where there was another girl involved, and SHE said they were married, and HE said she was a crazy ex, and she was stalking me and leaving threatening messages. I left. I cared deeply about the guy, but it wasn't worth it. Cheating wasn't the answer.