Genesis 6:1-4

And after that men began to be multiplied upon the earth, and daughters were born to them. The sons of God seeing the daughters of men, that they were fair, took themselves wives of all which they chose. And God said: My spirit shall not remain in man for ever, because he is flesh; and his days shall be a 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 brought forth children, these are the mighty men of old, men of renown.