My two cents: I believe that Sonny did have affairs, but probably did practice his version of faithfulness. Many mafia men had mistresses on the side, and I'm sure that Vito was the exception. And, yes, I'm sure that Sonny would have provided for Vincent, but I'm not sure he would've been welcomed into the family.

As for Fredo, I think that TB hit it right on the head. He was definitely kept close to his parents, and probably went a little overboard in Vegas. I think that the reason he never had children was that he was a child himself. How could he have handled the responsibility??

Michael was quite capable of being rather sensual - witness his relationship with Appollonia. However, his relationship with Kay was completely different. His marriage to her was based on the need for a wife and family and an air of legitimacy, although I believe he did love her very much.