I believe that the "sons of God' are speaking of the sons of the godly line of Seth and the "daughters of men" are the ungodly line of Cain.
descendants indicate that “sons of God” applies to them: they “began to call upon the name of the Lord”
(4:26), “walked with God” (5:24), and “found favor in the eyes of the Lord” (
6:8).
Second,
Genesis 5:1–3 restates that Adam was created in God’s image and that Seth was begotten in
Adam’s image. The implication is that the line of Adam through Seth constitutes the sons of God.
Third, the line of Cain is pictured in exactly the opposite light. Cain is driven out from the presence of
God. Lamech, a descendent of Cain, takes two wives, contrary to the standard established in the garden.
When Cain murders his brother, God confronts him with his deed; yet, when Lamech commits murder,
the absence of God is painfully obvious. Cain laments that his punishment is too great to bear, but
Lamech boasts that he is able to take care of himself — he doesn’t need God’s protection. The primary
characterization of Cain’s line is that they were becoming increasingly separated from God.
Fourth, the writer of
Genesis states that the sons of God saw that the daughters of humankind were
“beautiful” (Heb.
tob). This connects the daughters of humankind in
6:2 with chapter 4. There, in the
genealogy of Cain, the sons of Lamech are named and something is said about each one’s impact on
society; for example, Tubal‐cain was the forger of all implements of bronze and iron. Tubal‐cain’s sister,
Naamah, is named, but nothing is said about her. This should make the reader wonder why she was
mentioned at all, especially since women were viewed as lower‐class citizens. Interestingly, her name
means “beautiful.” This is not the same word used in
6:2, but we have already seen that passages do not
need to use the same terms to be parallel
This article gives a good presentation of that view:
http://74.125.47.132/search?q=cache...t+views+of+genesis+6&hl=en&ct=clnk&cd=6&gl=us
This article also gives good reasons why this passage is not referring to fallen angels mating with women on earth.
I had trouble copying and pasting excerpts in.