Personally, I think both woman represent different aspects to both men. Mama, was old school, she allowed Vito to go along with what he wanted to do, she was happy but submissive in her role, old school.

Kay on the other hand represents new ideals and changing times and not to mention was Michael's better half literally (his voice of reason). I think Michael married her because he did want to go legit, but the allure of the lifestyle and the knowledge with which he has in running a family always gets the better of him.