"What method of Bible interpretation allows us to assign a different definition to the “sons of God “in
Genesis 6?"
In the original creation all of the angels were considered to be "the son's of God"
However, when a portion of the same rebelled against the Lord, including Satan as the instigator, these are no longer considered to be the "sons of God"
This rebellion took place long before the Genesis 6 account, so the reference to the "son's of God" in the passage were not fallen angles, but human men ... and in this case men of renown and position who were polygamists and they took themselves many women
When men gain power, status, and wealth the more evil they become and they consider themselves to be above the law .... this is nothing new
These human "giants" of stature new better and were not ignorant of the Lord's suggested ways for human's to live .... but they became godless and reprobate .... engaged in behaviors that would satisfy their lusts and evil pleasures
I would suggest to reference a good Bible concordance for the meaning of the term "giants" used to describe the reprobate humans living on the earth during the days of Noah