Genesis 6 is a very interesting passage with many good Bible teachers rejecting the view that the sons of God refer to fallen angels, but rather refer to Sethites. But in my opinion, I think they do so because of their own human logic and at the expense of fundamental, exegetical practices that fail to truly provide explanations and definitions that come out of Genesis 6 itself and that also dance around certain New Testament passages like Jude 6-7 and 2 Peter 2:4f. The Jude passage is especially important in the light of the Greek text and the contrast Jude is making. The Sethites are never called sons of God, but there is plenty of evidence of angels being called sons of God.
I heartily agree with Bob Deffinbaugh’s arguments and presentation of this passage in his commentary, Genesis: From Paradise to Patriarchs which is on our web site in the “Bible Studies / Old Testament / Books / Genesis” section. I believe those who reject this view do so through exegetical gymnastics because of their own refusal to believe this could happen. They often refer to Christ’s statement about angels not marrying, but this is talking about God’s normal plan for them. They were created as a host and were not to propagate like mankind and fill the earth. This in no way says that under Satan’s orders and power they could not leave their own (idios, unique, peculiar, distinct, proper) domain (arche, rule, sphere of rule, influence) and abandon their own proper abode (oiketerion, habitation, dwelling place) (Jude 6).
Jude 7 simply reinforces this by the comparison with Sodom and Gomorrah (“just as…in the same way these indulged in gross immorality and went after strange [heteros, “another of a different kind” clearly identifying their departure as one from the laws of creation]). The comparison with S & G and their homosexuality provides a perfect illustration of this. Both the angels of Genesis 6 and Sodom and Gomorrah broke with what was natural to them.