Cristina,
Very interesting point. My take?

After Michael witnesses the destruction of his entire family, and loses everyone that is of importance to him (his brothers, his parents, his wives, estranged from Tom, even Rocco's dead) at the end of GF2, I believe that he is too emotionally bereft to commit to another relationship. His loss is so complete, with the exception of Connie and his children, that he cannot imagine starting life over with another woman. I think he feels that he gave his heart twice (although I don't believe he truly loved Kay when he married her, but that's for another thread), and both marriages ended tragically. I think he walled himself off from that sort of love, and decided to devote himself to his children.