I believe Michael really loved Kay at the beginning--before Vito was shot. You see him actually being playful with Kay in their hotel room in a deleted scene. But, after he killed Sol and Mac, and Sicily, and Apollonia was killed, Michael was a changed man. There's no romance in the scene where he woos Kay in New Hampshire--it was a business negotiation, Kay was another step in his obsessive quest for "legitimacy"--a WASP wife to bear his children and bolster his "legitimate" front. Whatever remaining feelings he had for Kay were subordinate to his selfish personal plans.