All the romantic love Michael had was drained from him when Appolonia died in the car explosion. His reunitiing with Kay was purely a matter of convenience. He'd been backk in the U.S. "a year, maybe more" before he even bothered to reach out to her. I think for both of them the marriage was really an arrangement. Their "courtship" on the street in New Hampshire was really more of a negotiation. Both of them were getting older (especially for those times) and both of them wanted families .. albeit for very different reasons.

From the beginning of GFII Kay seems to be trying to find a way out. Before the attempted hit, she reminds him that he had broken his promise to make the Corleone family completely legitimate within five years. everyone agrees what that look on her face after the hit meant. Then there is the scene where she wants to eave the Tahoe compound and Hagen basically tells he she is a prisoner.

Getting divorced in the 1950's was no easy or socially acceptable thing. For Kay, to take on Michael was literally a matter of life or death, so she had to be very careful. Even after she left him, he took the kids from her and wouldn't even allow visitation.