He found something shiny and new that distracted him from the importance of a REAL relationship w/ someone who cares about you. And then he treated you as his backup. When she dumped his ass, he went running back to what he knew.
I think you should take a long, hard look at what you will and will not accept from a mate in a relationship. If you do keep him, let him know that he burned his get out of jail free card, and there will not be a next time.
"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."