Spouses that want to cheat don’t necessarily need their wives or husbands to give them an excuse. Cheaters make up their own excuses for their infidelity. And yes, even really good spouses get cheated on.
Affairs are typically acts of anger -- he had built up a ton of resentment toward you, and since he's an avoidant, he hadn't given voice to any of it or worked any of it through. I'm not saying that you deserved his resentment, it could be completely irrational, but the point is that it exists. People typically leave relationships for 1 of 2 reasons. Loss of attraction or they do not see a happy future together.
LH, What you say may be right for the average case but this is clearly not a "typical" case since her exH slept with someone else the night before his wedding.
I also find that sometimes your message gets lost because the conversation devolves into an argument such as what happened with T and KML above. Perhaps you can recognize when this happens and employ your active listening and validation skills instead? You make a lot of good points and I would hate for your messages to be lost because of the delivery.