LW, First of all, Happy Birthday!!! Hope you have many, many more.

As for Michael and Kay, I believe that Michael (an the novel may go into more detail) felt that the assassination attempt on his father brought the Family into the media spotlight, such as it was then. I think in the novel, he even tells Kay that there might be reporters at the hospital when she wants to visit Vito.

I believe Michael was afraid that Kay would be dragged into the 1940's equivalent of a media frenzy, and he didn't want that to happen. At the wedding, she obviously didn't have a full grasp of what his family (and Family) was truly about, and I don't think he wanted her to really find out.