I believe Michael loved Kay, in his own way. But that way wasn't very warm and passionate. The courtship scene (with Michael looking like a pint-sized banker in that Homburg hat) sounded more like a business negotiation than a proposal of marriage. Not for nothing did Kay ask, "Michael, what do you want from me?"
On the other hand there's the deleted bed scene which shows them both as a loving couple, very warm, very playful.
Something changed then. I think, it's not only that he lost his warmth for Kay, after Appollonia's death he lost the ability to love anybody.
