I'm sure he probably has. I also know that he and I were running pretty intense EAs simultaneously. Mine never became physical, I don't know if his did. I know they are still in touch, but it is very sporadic (one message every six months; living in different countries), I have no contact now with my EA partner. I don't know what to say; I learned to separate my own self-esteem from fidelity very quickly. I guess we have an open relationship in a closed setting. To the poster who claimed that "anyone with integrity or honesty would never cheat": alas, both our careers are pretty much dependant on our integrity and honesty. But you can divide your professional ethics from your personal ones.
I agree with all pp's about what might lead you to this place: for me, it was all about attention. Someone actually bothering to remember something I'd said to them the day before. Someone actually curious about me, what made me tick. Again, I get it, the MNers will be saying, why do you actually stay with someone who isn't curious about you, who doesn't care about what makes you tick? I realise, it's in fact quite shocking the degree to which he literally has never given a fuck about me, my past, my interests, my life. And yet: on one level, we understand each other, we get along day-to-day well, we are united in dealing with the challenges presented by one of our children.
My EA was because I'd found someone who understood, listened, remembered, cared. His EA was because he'd found someone who was from his world, and so understood his references, his concerns, his past, in a way I just cannot bring myself to care about. We're reaping the whirlwind of having sowed a past where it was okay that we came from totally different backgrounds, had a huge age gap, etc. etc.
In other words, if we were prepared to accept this very odd coupling, we had to be prepared that it would lead to efforts to seek understanding elsewhere, when our own flexibility failed us.