What KML said. He could not find happiness within himself and it's easier to blame it on others. My ex gave me the same BS. Yeah, he married his A partner, and he "affaired down" so he could feel like the man.
For YEARS I figured it was me, I was unworthy, I was a bad wife, but you know what? I wasn't. I improved me for me, gained back some of my self respect and confidence he destroyed. I can find happiness within now, and he can only find happiness from external sources, and he continues to blame them when he isn't happy.
I have also met a bunch of DB'ers in person, some are very close friends now and we spend time together, my kid has met them I am friends with their friends too, and like KML said, very down to earth, attractive, successful, head on straight loving people. Who were cheated on and blamed for it. Anyone who would leave these people were not happy with themselves.
I know it is an instinct to take this on as you making him unhappy, but please don't.