Considering Genesis 6 though everything we are thinking is a moot point though because now it doesn't matter at all if the daughters of men are descendants of the blood line of Cain or of Seth's. Adam is the son of God,,Adam is also both the daddy of Seth and Cain therefore both Seth and Cain are sons of God in as much Cains sons could have seen the daughters of Seth and took them to wife and bore the Giants.
Except theres a spiritual application, that the sons of God are those who have an inheritance from God, and Cain to me rejected God and killed Abel, and rejected his inheritance. Noah was a preacher of righteousness and the world rejected him which means they rejected God. So only Noah was left as a faithful Son of God by the time of the flood, all the other descendants of Seth had intermingled and died and the descendants it seems have taken on the Canaanite Idolatry. Same thing happening today.
Romans 8:14 For all who are being led by the Spirit of God, these are sons of God.
I have a different way of looking at things than most, for example pre-flood I think people see giants as an oddity, yet I see the world as populated by giants and Noah being the odd one out, what I mean is for example say the average height of men was 7 foot, and then Noah was only 5 foot freak of nature.
Just speculation but I also think most men had six fingers including Adam to me it makes sense for the number of man is six, yet Seth was born with five so im guessing at least Noah and his wife were born with only five fingers on each hand which Im guessing was a rarity, yet because he and his descendants were the only survivors its now the Norm. (Ild also guess Hams wife had six fingers)
Where Im going is that there are a lot of people who are still born with six fingers so, how ever it happened the Nephilim DNA or Cain's genetics appear to be still quite strong in the people dwelling in Canaan after the exodus and even in the entire world today, as people are once again getting bigger.
So I can accept the possibility that I could be a direct decendant of both Noah and Cain, which I see as the Nephilim lineage yet it makes no difference to me as its belief in the Lord is all that matters.
Im just saying this so you can see where im coming from, theres no way I expect any one to agree with such a weird outlook, but hey everyones idea of what life was like pre flood is a weird guess in my humble opinion.