While reading the NIV Study Bible recently, I came across some interesting notes on Genesis 6:1-4, concerning the situation before the Flood in which the "sons of God saw the daughters of men were beautiful, and they married any of them they chose."
From what I had always understood (mainly by reading WTS literature) these "sons of God" were angels that sinned by taking the daughters of men as sex partners. However, the NIV Study Bible, and some other bible commentators, mention that there were other possibilities:
1. That the "sons of God" referred to in Genesis 6:2 were actually decendants of Seth who chose to marry decendants of Cain. Some of the reasoning on this (from what I've gathered from a couple of online Bible commentaries) is evidently based on Hebrew writings stating that Seth's decendants took to living in a mountainous area, while the decendants of Cain took to living in a valley area. In time, according to these stories, the Sethites took some of Canaanite daughters as wives. The idea of sons of Seth as "sons of God" comes primarily from Genesis 5:26, which states that after the birth of Seth's son Enosh "At that time men began to call on the name of the LORD". Though the WTS has always postulated that this was a derogatory type of name-calling, the NIV and other commentaries seem to portray it in a positive light.
2. That the "sons of God" may refer to men of some 'royalty' at the time, who, taking after the boastful Lamech (Genesis 4:19-24), carried on polygamy and established harems for themselves.
Included in the above reasoning is the assumption that angels could not have desired the daughters of men, with the verse at Matthew 22:30 (Jesus' words that the resurrected will be like "angels in heaven" and thus not be married).
However, the NIV does point to Jude's words in Jude 6,7 concerning angels that "did not keep their positions of authority but abandoned their own home" as possibly supporting the idea that it was indeed angels who were referred to in Genesis 6:2. On the other hand, it can be noted that Jude does not specifically state what these angels did, and some commentators offer that their sin took place when they joined Satan's rebellion at an earlier time.
While fallen angels are also mentioned at 2 Peter 2:4, again their specific deeds are not mentioned. Despite the fact that the next verse comments on the judgement of God in Noah's day, again most commentators' works that I've checked do not make an association between the two. So I'm not totally sure whether 2 Peter 2:4 confirms the supposed angelic act of cohabitation with the daughters of men recorded at Genesis 6:2. WTS literature of course always makes the association between the two verses.
I'm continuing to research this matter. Any thoughts or research or comments from anyone would be greatly appreciated!