Genesis 6 has always been a puzzle for biblical interpreters.
“When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. Then the Lord said, “My Spirit shall not abide in[a] man forever, for he is flesh: his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown.”
Who are the Nephilim? Is this teaching that angels were intimate with women? Are these demons? The default view of many conservative churches is that angels were having sex with women, however embodied they became. However, this a passage rich in biblical theological meaning and opens up for the bible student the basic narrative of Old and New Testament.
I recommend to you the program on Christ the Center interviewing Rita F. Cefalu, author of “Royal Priestly Heirs to the Restoration Promise of Genesis 3:15: A Biblical Theological Perspective on the Sons of God in Genesis 6″ published in the Westminster Theological Journal. It is dense and requires some playback to understand the point being made. But it is a great example of Biblical Theology, interpreting a passage in the context of the history of redemption. I am very thankful to Westminster Seminary for steeping me in Biblical Theology, especially per Geerhardus Vos’ “Biblical Theology: Old and New Testament.” By the way, Reformed Foum also has a series work its way through Vos’ book that is a great intro to this method of biblical interpretation.