Nah, he didn't regret it. Though at first he was doing it out of disdain for his father's "business," Kay was still the sort of woman that was not about to turn a blind eye to things. Of course he kept her out of the loop on a lot of other things as the years went by, but just the same, she knew the lay of the land and Michael did love her for her strength and intelligence as a sort of prototype "modern progressive" woman. The sort of woman who would demand to know certain things.

Appolonia on the other hand, was a completely different sort of woman, filling a traditional role. While Kay no doubt saw herself as an "equal" to Michael, Appolonia was clearly filling the traditional role of subordinate, tending to the needs of her King, who in turn strove to protect her and shield her from the evils of the family business, in much the same way we saw Vito do when he closed the door to open the parcel of guns in part II.