Back when Jimmy Carter was running for president, he said in an interview that he never cheated on his wife but he did "lust in his heart". In other words, he was saying that he had fantasies but never went further than that.
It's normal to indulge in fantasies, even after you are married. There's nothing wrong with that, as long as you leave it at that.
If you think there is a danger that you would cross the line the next time you and Michael IM each other, then don't allow yourself to converse with him again. Too many people start out innocently messaging but end up crossing the line.