Yeah that's not his why. That's just the excuse he told himself to justify it.
Don't you think some married people wonder about being with someone else, even just in fantasy, and especially if they've only been with one or two people?
I think that sounds normal to wonder but many people don't cross that line from fantasy to reality.
What allowed him to cross that line is his why. Many times it's several things, not just one personal flaw. What he told you isn't a reason, it's an excuse.
He's not digging hard enough. Most people need some therapy to get to the real reasons. If he's not in IC get him signed up.