AMR mentioned that the Nephilim merely mark the time in the passage and are not directly connected to the Sons of God. It seems to be that the Nephilim issue is sometimes a distraction from what the passage is actually talking about.
I hope you do not mind some commentary on the passage.
Gen 6:1 And it came to pass, when men began to multiply on the face of the ground, and daughters were born unto them,
Notice that the comment above is not "sons and daughters" or "children" were born, but only "daughters" are mentioned. I think the sons are intentionally omitted and this omission is significant. The 2nd verse elaborates on the issue of these daughters.
Gen 6:2 that the sons of God saw the daughters of men that they were fair; and they took them wives of all that they chose.
Here again the daughters are mentioned. In fact the daughters are stressed, they are fair. Now for the issue. The Sons of God takes daughters in the plural. In fact the plurality of the daughters is stressed in that the Sons of God took wives "of all that they chose."
Gen 6:3 And Jehovah said, My spirit shall not strive with man for ever, for that he also is flesh: yet shall his days be a hundred and twenty years.
The judgement of God is kindled at this behavior. I am aware that some take this judgement to be the result of men and angels uniting in marriage. Angels are not mentioned in the context, one must job to a different context and import a concept to define the Sons of God as angels. Some take it to be the Cainite line mixing with Sethites. While this is at least a more contextual because the lines are mentioned right before this, I still think there is a 3ird option that focuses upon the introduction of polygamy.
Gen 6:4 The Nephilim were in the earth in those days, and also after that, when the sons of God came unto the daughters of men, and they bare children to them: the same were the mighty men that were of old, the men of renown.
Here I want to cross reference this verse with Genesis 4:19-24
And Lamech took unto him two wives: the name of the one was Adah, and the name of the other Zillah. And Adah bare Jabal: he was the father of such as dwell in tents and have cattle. And his brother's name was Jubal: he was the father of all such as handle the harp and pipe. And Zillah, she also bare Tubal-cain, the forger of every cutting instrument of brass and iron: and the sister of Tubal-cain was Naamah. And Lamech said unto his wives: Adah and Zillah, hear my voice; Ye wives of Lamech, hearken unto my speech: For I have slain a man for wounding me, And a young man for bruising me: If Cain shall be avenged sevenfold, Truly Lamech seventy and sevenfold.
Some observations about Lamach... he is a violent man. He brags about killing a man who wounded him, and killed a young man for bruising him. Then he violently compares himself to Cain. Cain killed Abel, but Lamach killed 2 men. I think we have here the mighty Hercules, or some strong man who is called the sons of the gods. I believe in some of the Egyptian mythology, the Pharoah's were called the sons of the gods. The other thing I note about Lamach is the number of wives. He has two, Adah and Zillah. This is the first record of polygamy in the bible and it is in the line of Cain. So Lamach is a powerful, strong man, who commits polygamy. This is the kind of person that I think Genesis 6 calls the sons of God. They come in unto the plural daughters of men and take for themselves wives (not a wife).
In Genesis, one of the first commands not related to the tree of the knowledge of good and evil is the commands pertaining to marriage. Gen 2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Also, the judgement upon Cain for murder is an issue, otherwise there are few moral judgments and laws in the first Chapters.
The judgment of the flood in Genesis 6:3 would then be because God saw this murderous and polygamous culture of mankind. The sons of God are violent men who take plural wives. The Nephilim may have been some of the sons of God, but not necessarily so. They merely mark the time of these events.
I know this is a very minority position that is not well known. I have always liked it because of the links to the context. It seems very contextual to me.