I believe Michael loved Kay before he fled to Sicily. I don't think he ever stopped loving Kay. But, he married Apollonia because he was "hit by the thunderbolt," and she was there, in Sicily, and Kay was far away. And he still loved Kay After Apollonia was killed and he returned to America. But Michael was harder, less sentimental, more calculating. A big part of his renewing his relationship with Kay was his belief, at that time, that he really would be "legitimate" in a few years. Kay, as a WASP from an old-line, respectable New England family, would help legitimize him and his kids. In the novel, Michael is "disappointed" when Kay converts to Catholicism--he liked her Protestantism because it was "more American."