You're begging the question by assuming that "sons of God" are godly, "daughters of man" are ungodly, whereas the Scripture shows the opposite - sons of God were being judged by God in Ps. 82:1-6, and they were satanic, as they presented themselves before God with Satan. (Job 1:6)
You do realize that Ps. 82:1-6 came
much later than Gen. 6, yes? What does Job 1:6 have to do with anything? You can't just go pulling things out of context and making connections without warrant for doing so.
All that is revealed is Noah's genealogy. I know what you're gonna say: "men began to call on the name of the Lord." But that's exactly the argument I'm making - just because men called on the name of the Lord in Enosh's generation doesn't mean they continued to do so in Noah's generation because "calling on the name of the Lord" is not a hereditary trait.
It has nothing to do with being a hereditary trait, so I don't know why you keep making that argument. Godly people have a better chance of raising godly people, and on down the line.
First, in Genesis 4, we see the line of Cain:
Gen 4:17 Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch.
Gen 4:18 To Enoch was born Irad, and Irad fathered Mehujael, and Mehujael fathered Methushael, and Methushael fathered Lamech.
Gen 4:19 And Lamech took two wives. The name of the one was Adah, and the name of the other Zillah.
Gen 4:20 Adah bore Jabal; he was the father of those who dwell in tents and have livestock.
Gen 4:21 His brother's name was Jubal; he was the father of all those who play the lyre and pipe.
Gen 4:22 Zillah also bore Tubal-cain; he was the forger of all instruments of bronze and iron. The sister of Tubal-cain was Naamah.
Gen 4:23 Lamech said to his wives: “Adah and Zillah, hear my voice; you wives of Lamech, listen to what I say: I have killed a man for wounding me, a young man for striking me.
Gen 4:24
If Cain's revenge is sevenfold, then Lamech's is seventy-sevenfold.” (ESV)
Lamech was evil, in the way of Cain but much worse. And, yes, that contrasts with Seth's line:
Gen 4:25 And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.”
Gen 4:26 To Seth also a son was born, and he called his name Enosh.
At that time people began to call upon the name of the LORD.
Gen 5:22
Enoch walked with God after he fathered Methuselah 300 years and had other sons and daughters.
Gen 5:23 Thus all the days of Enoch were 365 years.
Gen 5:24
Enoch walked with God, and he was not, for God took him.
Gen 5:25 When Methuselah had lived 187 years, he fathered Lamech.
Gen 5:26 Methuselah lived after he fathered Lamech 782 years and had other sons and daughters.
Gen 5:27 Thus all the days of Methuselah were 969 years, and he died.
Gen 5:28 When Lamech had lived 182 years, he fathered a son
Gen 5:29 and called his name
Noah, saying, “Out of the ground that the LORD has cursed, this one shall bring us relief from our work and from the painful toil of our hands.” (ESV)
Gen 6:8 But Noah found favor in the eyes of the LORD.
Gen 6:9 These are the generations of Noah. Noah was a righteous man, blameless in his generation. Noah walked with God. (ESV)
Right from the start Cain's line was evil and Seth's line "began to call upon the name of the LORD."