My take, for what it is worth!
Both Michael and Kay deluded themselves

“when he told Kay that the Family would be completely legitimate in five years” as Evita posted “he truly believed” that 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

Coppola and Puzo messing with us! these anomaly things....