That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
The increase of mankind is, in itself a blessing. Genesis 1:28. But see Proverbs 29:16
Observe the different expressions: sons of God, and daughters of men. If you turn to Genesis 4:26 you there discover that the children of Seth are said to call on the name of the Lord; including both sons and daughters; and hence, therefore, these are meant by the sons of God. See 2 Corinthians 6:18; Galatians 3:28. And if you turn to Genesis 4:16, then Galatians 4:19 and then compare both with Genesis 2:24, you will discover that the posterity of Cain, both by departing from God, and throwing off the reverence due to his Divine authority in the institution of marriage, by a plurality of wives, are they which are thus distinguished as the daughters of men. See, also, in the further confirmation. Matthew 19:3-9; 2Pe 2:14; 2 Corinthians 6:14-15; Exodus 34:15-16.