The account begins with mankind multiplying and giving birth to daughters. Born among mankind were sons and daughters. The question is who were "the sons of God" and "the daughters of men"? According to Gen 4:26 from the lineage of Seth a human son was born, and he was called Enos. It was then that mankind began to call upon the name of the Lord. Is this enough to prove the line that originated from Seth, the son of Adam, who was another son appointed by God instead of Abel whom Cain killed, were human sons of God who procreated and produced offspring with daughters of men from the line of Cain? Or the line that introduced revenge and murder to mankind?
Genesis 4:25-26 (KJV) And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew. And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the LORD.
From this passage we learn that calling upon the name of the Lord did not come from the line of Cain, who had been driven away from the line of Seth to become a vagabond upon the earth. It would be unreasonable to deny those who call upon the name of the LORD are not the sons of God. But who are the daughters of men these sons of God desired and took against the will of God?
Genesis 4:3-5 (KJV) And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
Offerings were obviously made unto the LORD even before mankind began to call upon His name. Both Cain and Abel gave offerings, but only the offering made of the firstlings of his flock and of the fat thereof by Abel were acceptable to the LORD. The offering of the fruit of the ground by Cain wasn't even said to be the firstfruit that was what the LORD desired.
The story goes on to show how in a rage of jealous anger Cain murdered his brother Abel and was subsequently driven out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. Cain's wife conceived and bare Enoch, he built a city and called the city after the name of his son, Enoch. As this line continues, we read of Enoch's son, Lamech who was a murderer and considered the vengeance of God toward Cain would be light compared to his own (see Gen 4:23-24).
The point for this rambling is that a strong case can be made that the daughters of men were women in unbelief, being born in an atmosphere without being in the presence of the LORD, and no desire for calling upon His name. They were daughters of wickedness, and though beautiful to look at forbidden of those who called upon the name of the LORD, the sons of God to marry. In disobedience to God the faithful sons of God married and bare children of wickedness through these daughters of men. The resulting flood that God brought upon the earth was to rid the world of this great wickedness of mankind, not of fallen angels. It is mankind in unbelief, without desiring to call upon the name of the Lord whose hearts are only evil continually, not fallen angels, if there be any such thing.
Genesis 6:1-5 (KJV) And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.