If we look at Gen. 5 we see the line of Seth. He was born after Abel was killed by Cain. IOW, the line of descendants from Seth are the sons of God. It is their marrying the daughters from the line of Cain the daughters of men that Gen 6 is describing.
That doesn't work for many reasons. No humans are "sons of God" without regeneration, the new birth. Seth's line was not regenerated.
The very language, "sons of God," "daughters of men" makes it wrong. You have to explain why the females in Seth's progeny are not also "daughters of men."
Why would the males in Seth's progeny, marrying the daughters of other men produce Giants ?
What would have been the danger to earth, the attack on the seed (Gene. 3:15), or the offense against God, should the males in Seth's progeny marry the daughters of any others?
The Scriptures never call Seth's progeny "sons of God," nore do the Scriptures ever call Cain's progeny, "daughters of men."
There is no particular curse upon the females of Cain's progeny.