Midrash su Genesi 24:4

You know this to be so from the fact that Abraham sought a wife for his son Isaac: And Abraham said unto his servant, the elder of his household, etc. (Gen. 24:2), and it is written also: I will make thee swear by the Lord, the God of heaven … that thou shalt go unto my country (ibid., vv. 3–4). Hence, it is apparent that Abraham was concerned with the observance of the law of Increase and multiply. Similarly, Isaac told Jacob: Arise! Go to Padan-aram (Gen. 28:2), to seek a wife proper for him. He did not wish him to be like the men of the generation of the flood, who wallowed in unchastity and indulged in sexual intercourse with species other than their own. Observe what is written concerning them: The sons of God saw the daughters of men … and took them wives, whomsoever they chose (Gen. 6:2). That was the reason they were obliterated from the earth.