In Piers the Plowman (9), William Langland recounts the story of the sons of God and the daughters of men. Though he extrapolates from the text, he gets the story right (I am quoting from the Penguin Classics edition, Piers the Ploughman (Penguin Classics) ):

“All Cain’s progeny came to an evil end. For God sent an angel to Seth [the Middle English reads, noght thi kynde with Caymes ycoupled ne yspoused], saying, ‘I command that your issue be wedded only with your issue, and never with Cain’s.’ But some ignored God’s command and coupled Cain’s children with Seth’s [Caymes kynde and his kynde coupled togideres]. Then God was angry with His creatures and said, ‘I will destroy man whom I have created . . . for it repenteth me that I have made them. And He came to Noah and said . . . . So through this accursed Cain, mister first came into the world - all because they contracted marriages against the will of God.”

Langland draws this lesson: “Good men should marry good women, even if they have no money. I warn all Christians never to seek to marry for wealth or rich relations.