You know, If I can bear grudge to FFC for the 1-st film, it must be for the way he screws up Mike's relationship with Kay.
In the novel the inner logic of actions, and decisions, is so elaborate that you cannot just take it and change to the opposite! Their relationship as it described by Puzo is a masterpiece, and what is most important, it is SHE who pursues him. It is she who finds him, and says before anything: "I don't care, I love you, I don't care.<...> You're really a gangster then, isn't that so? But I really don't care. What I care about is that you obviously don’t love me."
This is a very significant dialogue, it shows that Michael never in fact deceived her. And he never promised to make family legitimate. He said:
" If everything goes right , Corleone Family will be completely legitimate in about five years. Some very tricky things have to be done to make that possible." And he didn’t say that it was the chief aim of his life, and he would be very much upset if everything didn’t go right and it wouldn’t be possible. He was not so naïve, I strongly believe. And he didn't say that he could be legit himself... Only business. And maybe. IF everything goes right. Appreciate the difference!
And in fact it was no condition of marriage, he told it after she assured him that she didn't care who he was, because she loved him. That is a certain obligation, huh?