This leads to another point:
Having a goumad is a standard of behavior in the Mafia. But, as far as we know, neither Vito nor Michael played around. Prudes? I doubt it--they were probably too busy accumulating and using power to have time for mistresses.
TB, I just think that neither Vito nor Michael had any inclination to violate their marriage vows. Despite the fact that they might murder at the drop of a hat, there were "limits" to their immorality.