My take, both Michael and Kay deluded themselves However when Michael told Kay
Quote
I mean in five years, the Corleone Family is going to be completely legitimate
“He truly believed in what he was saying” that it would be so

Why would Michael need to lie to Kay to get her to marry him?

Michael had no qualms marrying Apollonia and surely could have married a nice Italian origin bride, suitable Mob Boss wife! instead of seeking Kay out