Referring to that mysterious passage early in Genesis: “When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive.” Understandably enough, but not what God wanted.
And they took as their wives any they chose. Then the Lord said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. The Lord saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. And the Lord regretted that he had made man on the earth, and it grieved him to his heart. So the <said, “I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them.”
It is the passage, as you may have guessed, that precedes the story of the great flood. Writing on the always interesting Calvinist website Reformation 21, the theologian Aaron Denlinger explains that Calvin thought the sons of God were the descendants of Seth and the daughters of men were the descendants of Cain, and that the first should have had more sense than to marry the second. (Other theories claim that the first were either angels or nobles. Calvin thinks that’s dumb.)
After discussing what this means for Calvin’s understanding of the Church, Denlinger explains what this means for Christian marriage:
All of this, of course, contains fairly obvious moral implications for believers in every subsequent age. “We are taught . . . in these words, that temperance is to be used in holy wedlock, and that its profanation is no light crime before God.” The profanation of holy wedlock consists in the sin — and it is, for Calvin, very clearly a sin — of marrying someone who does not belong to the people of God. It is a sin that will, Calvin thinks, inevitably lead to more sin and ultimately even apostasy: “It is impossible but that, in the succession of time, the sons of God should degenerate, when they thus bound themselves in the same yoke with unbelievers.”