As opposed to the acts of Fredo, Carlo, Tessio...what Kay did was more of a personal betrayal to Michael than something that would put the Family and the Corleone Empire in peril.

Hurt and horrified as he was, it was no reason to have her killed.

And by the way once she spilled the beans about her abortion, I do not think he loved her any longer at all. All one need do is watch the look on his face as he shuts the door on her, then turns away. Total disdain.

What we see in GFIII, made nearly 20 years later, is nothing to go by....the supposed
'reconcilliation' of Michael and Kay was made up solely to find a reason for Pacino & Keaton to have some sentimental scenes in the picture.

Aside from all that, I agree that Michael would not take from them the mother of his children.

Apple