Why would he leave you for someone uglier? She probably was willing to sleep with him. But, what does it matter if he left you for someone uglier?
You could just look at it in the time tried and true manner of: You dodged a bullet, be grateful he turned out to be an ass and left you now instead of later when it'd really hurt you financially and emotionally.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."