The purpose of the inspired writer of Genesis is not to record a comprehensive history of the world or to answer all of the questions we could possibly imagine. The early chapters of Genesis are not designed primarily to satisfy our curiosity but rather to teach us about God, mankind, sin, and salvation.
Historically there have been various suggestions for understanding the reference to “the sons of God” and “the daughters of men” in verse 2. Some have seen it as referring to incarnate fallen angels (demons) intermarrying with human women. But this seems highly unlikely since angels are spiritual beings that do not marry (Matthew 22:30). Others suggest that the “sons of God” were a line of demon-possessed kings following after Lamech in the line of Cain. The problem is that there is no mention of these kings in the text. A third option seems more likely. From Augustine down through Luther and Calvin along with most commentators, this view takes “sons of God” to refer to the descendants of Seth, the godly line whose genealogy was provided in the immediately preceding verses, and “daughters of man” to refer to the descendants of Cain, the line of unbelief and wickedness which was given at the end of chapter 4.
One of the chief sins of God’s people described in the Old Testament was the regular transgressing of God’s boundaries for marriage. Intermarrying with unbelievers and pagans was one of the characteristic ways that the people sinned against God. This sort of intermarriage was a grievous sin because it led God’s people into the idolatry and immorality of the nations such that “the LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually” (vs. 5).
And so, as we will see, God – consistent with his eternal righteousness – determines to judge the world. His judgement will be meted out against all creation whose corruption due to sin was total. And yet, God determined to preserve a righteous line through the man Noah who, we are told, “found favor [grace] in the eyes of the Lord” (vs. 8). This is first mention of grace found in the Bible. No matter how man may sin, God will never abandon his promise to save his people from their sin.