I think Michael loved Apollonia more, not that he didn't love Kay, just loved her more. Apollonia was an old fashioned girl and she and her family "approved" of Michael and his way of life. Michael always had to "sell" things to Kay: He didn't kill his sisters husband, make the family business legit, I can change, etc.
I think Michael saw Kay as a vehicle to give him children to keep his way of family going.
Michael was always pretty even tempered and level headed no matter what was presented to him, except for when Kay told him she had an abortion
Granted what person would not be beside themselves but his reaction showed me he cared more for the children she could bare than her.