Genesis 6:1-4

And it came to pass when mankind began to multiply on the earth, and daughters were born to them, that the sons of God saw the daughters of men that they were fair, and took themselves wives of all that they chose. And Jehovah said, My Spirit shall not always plead with Man; for he indeed is flesh; but his days shall be a hundred and twenty years. In those days were the giants on the earth, and also afterwards, when the sons of God had come in to the daughters of men, and they had borne children to them; these were the heroes, who of old were men of renown.