What would you think if a man says...

[Beezar]

...that the reason his marriage broke up after 20 years is because the relationship broke down a long time ago.

But then you learn that at the point of breaking up, he had another woman and is still with her.

YABU - you accept the other woman occured as the result of the broken down marriage
YANBU - you believe the other woman was the catalyst for the decision.

Don't over think it. What initially springs to mind?

Not ready whole thread but from your OP I'd think this is a man not to be trusted!
My late husband had been separated for a year from his wife of 20 years when I met him. I asked was he ready yet for a new relationship? He said it wasn't like he woke up one morning and wanted to leave, the marriage had been breaking down for at least four years and then she eventually asked him to move out. But neither were seeing anyone else - they were still emotionally involved and trying for quite some time. It was only when he left and a while had passed that he understood how much his feeling had changed and he was ready to start with someone new. Anyone who overlaps relationships has checked out of the first one for a considerable time.

Neither really, my main thought would be that he had glossed over his own wrongdoing, but I wouldn't switch to thinking he only left because of OW - the relationship probably was still shit.

Funny how so many men are in 'broken/non-existant' relationships but still sharing a home, finances, life and bed with their partners.

Take it with a freight train full of salt.

First option. That's not to say I wouldn't think the man was an arsehole for cheating on his wife.

Overwhelmingly goes in favour of the OW being the catalyst on the vote. Thank you all very much for joining in! I think H is going to have a hard time when it all comes out.

It pleases me. 😁

This is more or less the complete script, as others have said. It's part of rationalising the loss of something wonderful, by downplaying it,