It's actually pretty common. I think it's just easier for the wayward to demonize the betrayed and make them the enemy, rather than facing the cold, hard truth about themselves. For my ex, he'd rather blame me for well, everything instead of face the truth. He still wants to see himself as a good person, and good people don't walk out on their families. Therefore, it must have been someone else who drove him to it.
My ex was so delusional he actually told me my sister would see things his way and take his side, because after all, she knows what I'm like. Yes, as it turns out, I used to be married to an idiot.