Wasn't it "Friends" that had the "list"? Each partner could write down a list of a handful of people that if they situation ever presented itself, you were allowed to follow through.
True cheating is lying. And IMHO, lying to ANYONE is a show of a lack of respect for the person you're lying to.
And no, I've never cheated on a spouse or boyfriend, nor have a I ever been with anyone who was married. If they had a girlfriend...well, that I've done.