I think at the time that Michael and Kay were married, and for at least several years thereafter, Michael did love Kay...as much as he was capable of doing so. During the last scene between Michael and Vito, when Vito asked "Your wife and children, are you happy with them?", and Michael answered "Very happy", I think he was sincere.

Whether or not that love would qualify as "love" according to the standards of anyone else, particularly Kay herself, and especially as the marriage began to fall apart later, is a separate issue.