Sextus Julius Africanus (160-240 AD) is perhaps the first to radically reinterpret and misinterpret the Genesis 6 affair as pertaining to what has come to be known as the Sethite view which claims that the “sons of God” are the godly lineage of Seth (why only males and why all godly?) and the “daughters of men” are the ungodly lineage of Cain (why only females and why all ungodly?).
As for example chronicled in section II of The Extant Fragments of the Five Books of the Chronography of Julius Africanus, he wrote:
When men multiplied on the earth, the angels of heaven came together with the daughters of men. In some copies I found “the sons of God.” What is meant by the Spirit, in my opinion, is that the descendants of Seth are called the sons of God on account of the righteous men and patriarchs who have sprung from him, even down to the Saviour Himself; but that the descendants of Cain are named the seed of men, as having nothing divine in them, on account of the wickedness of their race and the inequality of their nature, being a mixed people, and having stirred the indignation of God.
At this point, a footnote states, “The text here is manifestly corrupt: e0pimixqe/ntwn au\tw=n, th\n a0gana/kthsin poih/sasqai to\n Qeo/n” to which I will say, whatever that means.
In any case, note that Julius Africanus’ claims are “in my opinion” and he provides no biblical backing for his opinion that from the descendants of Seth, apparently exclusively, righteous men and patriarchs sprung but the descendants of Cain, apparently exclusively, have nothing divine in them. In fact, we do not know that they, in general and as a lineage “stirred the indignation of God” but know only that Cain did so.
Julius Africanus ends this section thusly:
But if it is thought that these refer to angels, we must take them to be those who deal with magic and jugglery, who taught the women the motions of the stars and the knowledge of things celestial, by whose power they conceived the giants as their children, by whom wickedness came to its height on the earth, until God decreed that the whole race of the living should perish in their impiety by the deluge.
Thus, he was clearly aware of the common understanding of Genesis 6 and even touches upon extra biblical claims about the actions of these Angels as the Bible does not relate what they taught but apocryphal works, such as the Book of Enoch do.
Note just how the times, and personal opinions, have changed to the extent that Africanus can, in his time, note that “it is thought that these refer to angels” but in 1831 AD Adam Clarke (Commentary on the Bible) notes that, in his time, “there is a distinction made here between men and those called the sons of God, it is generally supposed that the immediate posterity of Cain and that of Seth are intended. The first were mere men, such as fallen nature may produce, degenerate sons of a degenerate father, governed by the desire of the flesh, the desire of the eye, and the pride of life. The others were sons of God, not angels, as some have dreamed, but such as were, according to our Lord’s doctrine, born again, born from above.” Clarke offers no elucidating data from the Bible’s contents, concepts and contexts but merely refers to the original and thus ancient interpretation as being something dreamed up.