The difference is history and circumstances. For the Kate AU, you have to keep in mind Cy never met Lynn in college, didn't become as socially developed, and key point here, never was introduced to kink. Even if Cy has a pretty strong libido across most universes, the reason he starts cheating on Lynn is because they were doing chastity kink play and he got frustrated at the fact denying her meant denying himself too, in a way. This version of Cy isn't kinky, doesn't see cheating as a "game" between Kate and him, so when she asks him to quit sleeping around if he wants to go steady with her, it's not a very big demand— they're not in a D/s relationship! As for the Divorce AU, it's less that Lynn threatened to leave, and more that she just up and decided to leave one day because Cy wasn't taking her emotional needs seriously, the cheating was just one part of it.