Michael's marriage with Kay: to begin with it did not seem he truly loved her after he came back from Italy, probably he wanted a wife and children out of tradition, and ego of a legacy after he's gone.
Years go by and his claim that he'd become legit were just talk, and Michael neglected his wife and children. More info. in Part 3.
Trivia, flashback ending scene: Marlon Brando initially agreed to return for the birthday flashback sequence, but the actor, feeling mistreated by the board at Paramount, failed to show up for the single day's shooting. Coppola then rewrote the scene that same day.