In the godfather why did Michael and kay divorce?

She couldn't bare the thought of her next born son growing up to be like his father and Michael got mad when he found out that she had an abortion when the whole time he thought she miscarried (because that's what Hagen told him), an argument starts and he slaps her for lying to him about the abortion. She tells him she doesn't like the "monster" he has become and claims she doesnt love him she left him.