This is from something I posted to a thread on eXTwitter:
If Genesis 6 is read as if it started at Genesis 6 and did not continue from Genesis 5, the wording of the first two verses of ch. 6 could be read as that the sons of God were not mortal humans, and many have so read it.
But chapter 6 is a continuation from ch. 5.
And chapter 5 is a continuation from ch. 4.
Chapter 4 tells about a division among the children of Adam.
Cain and Abel seem to have both started okay, but Abel received and accepted instruction from the Lord, and made his sacrifice accordingly. In contrast, from verse 7, we are given to understand that Cain, when he brought his sacrifice, did so at Lucifer's direction.
The Lord said, if you don't follow me, you won't get it right. And if you don't do right, you start into the path of sin. (If thou doest well, shalt thou not be accepted? And if thou doest not well, sin lieth at the door.)
And that's what Cain did. He ended up committing murder and he and his children left Adam and the rest, and no longer followed God. Some might ask why Cain was not subject to capital punishment. As I understand it, especially in the first generation of Adam's children, and since Cain did promise to refrain from further killing, God considered it better to let Cain continue to have children to populate the world.
When Seth was born, Eve expressed hope that this son would follow God, and v. 26 indicates that Seth and his children did follow God.
So we see that, among the children of Adam and Eve, some followed God and some didn't. In other words, some were children of God and some were not.
Chapter five is not much more than a listing of the lineage of Noah and his children – of interest to us among those who followed God because of Noah. These are not the only ones of the descendents of Adam who followed God, they are just those in the direct lineage of Noah.
Thus, when we get to verse 2 of chapter 6, we can see that what it is saying is that (unsurprisingly) some of the children of those who followed God (the sons of God) were seduced away into intermarrying and joining in with the children of those who did not follow God (the sons of men).
(I think we can safely assume that at least some of the intermarriage involved children of those who did not follow God going the other direction, joining with those who did follow God, even though it's not explicitly stated in Genesis.)
Looking back to chapter 4, those who did not follow God tended to focus on specialized (technological) skills instead of continuing to learn greater things from God. (See Jabal and Jubal and Tubal-cain.)
And we also see in both Cain and Lamech that Satan was already teaching people to do conspiracies. (This Lamech, of course, is a different person than Noah's grandfather.)
We do not need to doubt that God taught His followers – His children – all the things that the children of men learned in their specializations, and much more. Thus the children of the children of God knew more about nutrition, physics, etc. than the children of men, and were of great stature – men (and women) of great renown among them.
Great stature – knowing about nutrition and health, many of them would well have been of significant physically greater stature, much as better nutrition and health in the last century has led to a sudden generational increase in height and physical strength, with children towering over parents.
These children of the believers would had great knowledge from their parents, but, in many cases, not the ethics and morals to use that knowledge well and wisely.
We can see them constructing a society with all the evils that we have in our modern society. That's why it concerned God that theirs thoughts were continually evil.
And now we can begin to see why God might have found it necessary to send the a big slushball comet or asteroid to cause the floods, and start the world fresh again with Noah.
