Kay had nothing to do with Michael's motivation for legitimacy. As I and others have opined, Michael's concept of legitimacy was different from most peoples. Kay was simply an accoutrement; she was utilized by him to achieve his version of legitimacy.
Right, Oli.
Bringing in Appolonia adds to this interesting thread. I think Michael's decision to kill Sol and Mac was motivated by his perceived need to rescue his father from what appeared to be another, likely successful, attempt on his life. He may also at that point have decided (probably subconsciously) to finally stop rejecting his family and rejoin it ("I'm with you now, Pop"). But I don't think he committed himself to being a Mafia Don at that point.
I see his wooing of Appolonia, and the old-fashioned wedding, as a kind of affirmation of his Sicilian roots and his patrimony--maybe even a rejection of the WASP-y ways he'd adopted, which Kay personified. Had Sonny not been killed, and had Michael returned to the US not in a leadership role in the family, Appolonia would have represented another step in reconciling with his roots and his father. But, her and Sonny's murders put him in the Mafia, all the way. His quest for legitimacy was his way of dealing with his father's aspriations for him and his need to be the boss of a Mafia family. I think he loved Kay in his own way, but she was window-dressing for his quest for legitimacy. The wooing scene in New Hampshire was brilliantly done--not a hint of passion, more like a business negotiation, right down to Michael dressing like a pint-sized banker and being trailed by a limo.
