Genesis 6:1-4

And when men began to be multiplied upon the earth, and daughters were born to them, the sons of God, seeing that the daughters of men were beautiful, took to themselves wives from all whom they chose. And God said: "My spirit shall not remain in man forever, because he is flesh. And so his days shall be one hundred and twenty years." Now giants were upon the earth in those days. For after the sons of God went in to the daughters of men, and they conceived, these became the powerful ones of ancient times, men of renown.