WOW, Cristina, no Appollonia?? I don't know if I've ever heard anything I disagree with more (of course it's your opinion and you are quite entitled to it). Appollonia represented not only his love for something innocent and pure, but the beginning of an understanding and an embrace of Sicily and his roots. Long estranged by choice from The Family, trying desperately to be "American" (Iby League college, WASP girlfriend), his exile in Sicily begins his understanding of the life his father came from, how Vito came to be Don, and Michael's turning away from that college boy that he had been. It is his beginning down the path of a Don, and Appollonia is a huge part of that. She is a woman much like his mother, one that will unquestionably stand by her man and the life he has chosen.

IMHO, he no longer loves Kay, as he is no longer that boy. His marriage to her is more something that he feels he "should" do, settle down, have kids and so on. Kay is the logical choice, not a choice of the heart.