https://enduringword.com/bible-commentary/genesis-6/

A. The wickedness of man in the days of Noah.

1. (1-2) Intermarriage between the sons of God and the daughters of men.
   Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose.

a. When men began to multiply on the face of the earth: During these days of rapid population expansion (especially because of long lifespans in the pre-flood world), there was a problem with ungodly intermarriage between the sons of God and the daughters of men.

b. The sons of God saw the daughters of men: Many have believed the sons of God were those from the line of Seth, and the daughters of men were from the line of Cain, and this describes an intermarriage between the godly and the ungodly, something God specifically prohibits (Deuteronomy 7:1-4, 2 Corinthians 6:14).

i. But this approach leaves many unanswered questions: Why did this make God angry enough to wipe out almost all the earth’s population? Why was there something unusual about the offspring of these unions (Genesis 6:4)? The idea that these were believers marrying unbelievers doesn’t seem to fit the record of the text.

c. The sons of God saw the daughters of men: It is more accurate to see the sons of God as either demons (angels in rebellion against God) or uniquely demon-possessed men, and the daughters of men as human women.

i. The phrase sons of God clearly refers to angelic creatures when it is used the three other times in the Old Testament (Job 1:6, 2:1, and 38:7). The translators of the Septuagint translated sons of God as angels. Those ancient translators clearly thought sons of God referred to angelic beings, not to people descended from Seth.

ii. Jude 6 tells us of the angels who did not keep their proper domain, but left their own habitation. Jude goes on (Jude 7) to tell us they sinned in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh. Here in Genesis 6, as in Sodom and Gomorrah, there was an unnatural sexual union.