I like Pink on this. Reference in your mind Gen 3:15 and the picture story of the Woman and the Great Red Dragon of Rev 12:
“....It is a significant fact that
some versions of the Septuagint contain the word "angels" in Genesis 6:2, 4......These "sons of God," then, appear to be angels who left their own habitation, came down to earth, and cohabited with the daughters of men. Before we consider the outcome of this illicit intercourse, let us first enquire into the cause of it.
Why did these angels thus "sin" (2 Pet. 2:4)? The answer to this question leads us into a mysterious subject which we cannot now treat at length:
the "why" finds its answer in Satan.
Immediately after that old serpent, the Devil, had brought about the downfall of our first parents, God passed sentence on the "serpent" and declared that
the woman’s "Seed" should "bruise his head" (Gen. 3:15). Hence, in due course,
Satan sought to frustrate this purpose of God. His first effort was an endeavor to prevent his Bruiser entering this world. This effort is plainly to be seen in
his attempts to destroy the channel through which the Lord Jesus was to come.
First, God revealed the fact that the Coming One was to be of human kind, the woman’s Seed, hence, as we shall seek to show,
Satan attempted to destroy the human race. Next, God made known to Abraham that the Coming One was to be a descendant of his (Gen. 12:3; Galatians 3:18; Matthew 1:1); hence, four hundred years later, when the descendants of Abraham became numerous in Egypt Satan sought to destroy the Abrahamic stock, by
moving Pharaoh to seek the destruction of all the male children (Ex. 1:15, 16). Later, God made known the fact that the Coming One was to be of the offspring of David (2 Samuel 7:12, 13); hence, the subsequent
attack made upon David through Absalom (2 Samuel 15). As, then, the Coming One was to be of the seed of David, He must spring from the tribe of Judah, and hence the significance of the divided Kingdom, and the attacks of the Ten Tribes upon the Tribe of Judah!
The reference in Jude 6 to the angels leaving their own habitation, appears to point to and correspond with these "sons of God" (angels) coming in unto the daughters of men. Apparently, by this means, Satan hoped
to destroy the human race (the channel through which the woman’s Seed was to come) by producing a race of monstrosities. How nearly he succeeded is evident from the fact, that with the exception of one family, "all flesh had corrupted his way upon the earth" (Gen. 6:12). That monstrosities were produced as the result of this unnatural union between the "sons of God" (angels) and the daughters of men, is evident from the words of Genesis 6:4: "There were giants in the earth in those days." The Hebrew word for "giants" here is nephilim, which means fallen ones, from "naphal" to fall. The term "men of renoun" in Genesis 6:4 probably finds its historical equivalent in the "heroes" of Grecian mythology.
Satan’s special object in seeking to prevent the advent of the woman’s "Seed" by destroying the human race was evidently an attempt to avert his threatened doom!"
http://www.pbministries.org/books/pink/Gleanings_Genesis/genesis_11.htm