¹ When humanity began to multiply on the face of the ground, and daughters were born to them, ² the sons of Elohim saw that the daughters of humanity were good. They took wives for themselves from them as they chose. ³ And YHWH said, "My spirit shall no longer remain with humanity forever, because they are flesh. His life span will be 120 years. ⁴ The fallen ones were on the earth in those days (and also afterward) when the sons of Elohim had intercourse with human daughters and bore offspring for them. They are the warriors, from eternity called the men of a name. (6:1-4)

    Certainly one question that jumps out of this text is, Who are the sons of God? Some interpreters have suggested they are the offspring of Cain, and that this story records the interbreeding of the lines of Cain and Seth. This would represent a mixture of the good and the bad lineages. This view is probably mistaken. Parallels to the phrase "sons of Elohim" in biblical and extra-biblical literature strongly suggests that they are divine creatures, commonly identified as angels (for example, see Job 1:6; 2:1; 38:7). They appear to have been errant members of the Divine Council, the body of angels who rule the universe with God. According to 6:1-4 certain of these angels were sexually attracted to human women and sired a race of giants.
    The possibility of such interbreeding defies human conceiving (pun intended). Were it possible, presumably such interbreeding would have resulted in humans acquiring the immortality of divine creatures. Probably for this reason God took steps to limit the longevity of humanity to a maximum of 120 years.

Rebel Angels, by Pieter Bruegel the Elder (1525-1569) Musée des Beaux-Arts, Brussels -- WebMuseum, Paris

    Fantastic and strange as the incident may seem, it plays an important role in the narrative scheme. The Yahwist used the incident to explain why God was finally moved to action. Sin had evolved so far as to infect the relationship between the divine and the human realms. The proper distinction between heaven and earth was no longer being maintained.

Myth. Many authorities see the survival of an early myth about the gods and humanity in this story. Kilmer (1987) notes the parallels between the "fallen ones" and the pre-flood apkallu sages of Mesopotamian tradition. The biblical tale also recalls those notorious incidents of divine-human intercourse in Greek mythology, as when Zeus bedded Io and Europa.