(as you said)

I too reckon WASP is our doing!
Besides WASP Kay was already Michael's girlfriend, long before Michael having to become the Don and the quest for "legitimacy"

Maybe both deluded themselves but he didn't lie to Kay to get her to marry him There was no need
As Michael had no qualms marrying Apollonia, there was nothing stopping him from marrying a nice Italian origin bride, suitable Mob Boss wife! instead of seeking Kay out

Kay too had no qualms and was going to live her rich and privileged life as Mob Boss' wife for those five years