Ewan MacColl used to tell it as a folktale. The young man kept telling his father that he was courting such & such a girl, but his father would always tell him "Nay, ye cannae wed her. She's ma ain natural daughter." Eventually his mother asked him why he was so sad. He told her, adding that he really loved the most recent girl. "Pay him nae mind lad," his mother said, "and just ye wed the lassie. He's no' your fayther at a'!"

I have been hunting this in the Aarne/Thompson Types Of The Folktale index. Will come back with its # if I find it.

~M~