I'm not surprised. As Michael got older, he got colder and more calculating, more manipulative. I believe Michael loved Kay before Sicily, and definitely was "hit by the thunderbolt" by Apollonia. But after she was killed and he returned to the US, he had given in completely to his hard-edged, cold, calculating nature. I think he may still have loved Kay at that time, but he was primarily interested in her as a symbol of his grand scheme for legitimacy: a WASP from a respectable family who would bear his children and make him look like a regular businessman. After he threw Kay out, he basically had no more time--or feeling--for a wife.