I've never understood while Kay married Michael either. He was back in America for over a year or so before he decides to contact her after not even letting her know he'll be out of the country for a long while. At their last encounter before that, he wanted nothing to do with the family business, as far as Kay was concerned. She was even relunctant to get back with him at first, saying it was too late. He told her the Corleone Family would be completely legitimate in 5 years. She should have made him wait 5 years to prove it before commiting her life to him. Since she didn't, she knew darn well what kind of life she may be getting herself into.