I think Michael really did love Kay for herself before he got involved in the family business. We see this in a deleted scene before Vito was shot, when he and Kay are cavorting in a NYC hotel bedroom, and he asks her to impersonate a telephone operator so he can tell Tom he's still in New Hampshire. The next time he sees Kay, they're having dinner in that hotel room before he goes to the hospital--no more cavorting. But, after he shoots Sol and Mac, no more Kay--no hint he's even thinking of her in Sicily--and it's all Appolonia.

He waits a year after returning to the US before "wooing" Kay in New Hampshire. That scene looks more like a corporate merger or acquisition negotiation than a marriage proposal. He tells Kay he loves her with all the passion of a lawyer scrutinizing a contract. Kay even says, "I don't know what you want from me, Micahel." I think he simply wanted to use her--to bear his children, to help give him a "respectable" front, and to validate his fantasy of being "legitimate." The novel even says that Michael was disappointed when Kay took instruction in the Catholic Church--he wanted his family to be "more American."

At his party in III, Michael shouts at her that "I spent my life protecting my family from the horrors of this world." "But you became my horror," Kay replies. She says she "dreads" him. That says it all. But, near the end, he's still using her--he asks her to "forgive" him after giving her a long rationale about his father, the family, etc. "You mean, forgive you, like God?" she replies. Then, improbably, she says, "I still love you, Michael, I always will." It's one of the weakest moments in the movie.