I believe the "Sons of God" in this passage are wicked human rulers (city kings) who considered themselves divine. They used their control to take women from their subjects by force (whether they were virgin or married).
http://davelivingston.com/sonsofgod.htm
One of the book of Jasher candidates explains it this way:
"Jasher" 4:16-19
16 And all the sons of men departed from the ways of the Lord in those days as they multiplied upon the face of the earth with sons and daughters, and they taught one another their evil practices and they continued sinning against the Lord.
17 And every man made unto himself a god, and they robbed and plundered every man his neighbor as well as his relative, and they corrupted the earth, and the earth was filled with violence.
18 And their judges and rulers went to the daughters of men and took their wives by force from their husbands according to their choice, and the sons of men in those days took from the cattle of the earth, the beasts of the field and the fowls of the air, and taught the mixture of animals of one species with the other, in order therewith to provoke the Lord; and God saw the whole earth and it was corrupt, for all flesh had corrupted its ways upon earth, all men and all animals.
19 And the Lord said, I will blot out man that I created from the face of the earth, yea from man to the birds of the air, together with cattle and beasts that are in the field for I repent that I made them.
http://davelivingston.com/sonsofgod.htm
One of the book of Jasher candidates explains it this way:
"Jasher" 4:16-19
16 And all the sons of men departed from the ways of the Lord in those days as they multiplied upon the face of the earth with sons and daughters, and they taught one another their evil practices and they continued sinning against the Lord.
17 And every man made unto himself a god, and they robbed and plundered every man his neighbor as well as his relative, and they corrupted the earth, and the earth was filled with violence.
18 And their judges and rulers went to the daughters of men and took their wives by force from their husbands according to their choice, and the sons of men in those days took from the cattle of the earth, the beasts of the field and the fowls of the air, and taught the mixture of animals of one species with the other, in order therewith to provoke the Lord; and God saw the whole earth and it was corrupt, for all flesh had corrupted its ways upon earth, all men and all animals.
19 And the Lord said, I will blot out man that I created from the face of the earth, yea from man to the birds of the air, together with cattle and beasts that are in the field for I repent that I made them.