David Clines has made available a paper he wrote in 1972 on the unusual story found in Genesis 6:1-4 about “the sons of god(s)” who had sex with “the daughters of men” and sired Nephilim (the “heroes of old”, the “warriors of renown”).
The Signiﬁcance of the “Sons of God” Episode (Genesis 6:1-4) in the Context of the “Primeval History” (Genesis 1–11), originally published in Journal for the Study of the Old Testament 13 (1979): 33-46.
Scholars have made a few different suggestions regarding the meaning of the phrase “sons of god(s)” (בני האלהים). Does it refer to demigods or angels? or to the Sethite line of human beings (in Genesis 5) in contrast to the Cainite line (of Genesis 4), or vice versa? or to human rulers or princes? In general, there is a debate as to whether “sons of god(s)” refers to divine or human entities.
Anticipating an issue which has come up in the current debate about monotheism and polytheism in ancient Israel and classical and later Greece and Rome, David Clines suggests that this split between human and divine options may not be so clear-cut, in particular with respect to rulers, and even more in particular to antediluvian rulers:
[T]he author of Gen. 6.1-4 in its present form did not work with a system of closed categories in which ‘sons of God’ must be either human or non-human. Are the בני האלהים here then both divine beings and antediluvian rulers?
Interpreted this way, the strange episode does not appear to be such an intrusion into the Primeval History (Genesis 1-11). Clines goes on to document other connections that he sees Gen 6:1-4 as sharing with the remainder of Genesis 1-11 and with the following Flood Narrative (Genesis 6:5-9:17).