As a friend of mine used to memorably put it, "you can't say you slipped and landed balls-deep in someone else's cooter."

Yep. The answer to "I'm not attracted to my partner" and "I want to cheat" is always the same - either talk to your partner and work through it, or be up-front about the fact that this relationship is done and move on. There's no call for unnecessary cruelty, either by slinging malicious remarks or cheating first and *then* leaving.