The margin notes of the KJV are there because alternate translations of a word or words is possible. Often times, as scholars have noted, the margin notes are to be preferred
I am not a fan of the KJV because I have found many instances of bias in their renderings, particularly with regard to the trinity. It is not a reliable translation but treated by some as if it came straight from God.....I strongly disagree with its renderings.
The margin note says:
call upon the Name of the LORD:
Or, to call themselves by the Name of the LORD
"men began to call upon the name of the Lord — rather, by the name of the Lord. God’s people, a name probably applied to them in contempt by the world." JFB
יְהֹוָה בְּשֵׁם קָרָא, literally to call in (or by) the name of Jehovah -- Keil and Delitzsch
"The marginal reading is, Then began men to call themselves by the name of the Lord; which words are supposed to signify that in the time of Enos the true followers of God began to distinguish themselves, and to be distinguished by others, by the appellation of sons of God; those of the other branch of Adam’s family, among whom the Divine worship was not observed, being distinguished by the name, children of men." - Adam Clarke
The LXX translates it as: "And Seth had a son, and he called his name Enos: he hoped to call on the name of the Lord God. "
Some translations use the word "profane" as does the ISV: "Seth also fathered a son, whom he named Enosh. At that time, profaning the name of the LORD began. "
It is therefore quite possible that the "sons of God" mentioned in Gen 6 refer to the men who had taken upon themselves God's name as opposed to those who did not.
I’m sorry but that is an extremely weak argument.
The rest of scripture backs up the fact the “the sons of God” in Genesis 6 were angels who had followed Satan into rebellion and used their materialised flesh to indulge themselves in very unnatural and depraved behavior for spirit beings. Sex and violence are their trade mark.....who rules the world? (1 John 5:19)
These rebels became the “spirits in prison” to whom Jesus went to deliver God’s judgment against them. God had put them under restraint, keeping them in spiritual darkness and taking away their power to take on flesh. Only faithful angels could take on human form after that. (e.g. Gabriel’s visit to both Mary and her cousin to inform them of their privilege to bear special children)
Your explanation leaves way too many questions unanswered.....
Most especially, why God found it necessary to wipe out every creature who had the breathe of life in them. If he just needed to punish the humans, why not do what he did to Sodom and Gomorrah? Why get Noah to spend decades of hard work to build an enormous ark to preserve life?
It is obvious the these special circumstances warranted special measures.