Another possibility (admittedly, a reach): Michael had a rare moment of objective introspection and realized that he could never be in a satisfying marital relationship after Kay, given the kind of life he led. He may have realized that no woman would have been satisfied (much less happy) sharing his life. I'm basing this on his reply to Senator Geary, after the "oily hair/silk suit" rap: "Senator, we're both part of the same hypocrisy [emphasis added], but never think it has anything to do with my family." The fact that he was willing to acknowledge his facade as a "legitimate" businessman as "hypocrisy" astounded me--even though he tried to exempt his family from it.