- I think “sons of God” are distinguished from “the daughters” (of mankind), not “mankind” as you say. Mankind is a genitive and hence a descriptive noun, not a primary noun in the phrase.
(2) The term “son of God” is used in ancient literature and the Bible to mean three things (not just one): angels, men in covenant with God, kings. For occurrences of sons of God referring to men (standing in a covenant relationship with God) see Deuteronomy 14:1; 32:4-5; Psalm 73:15; Hosea 1:10; Romans 8:14,19; Philippians 2:15; 1 John 3:1-2. The term “son of god” is also an ancient expression for human kings or rulers. For an exact parallel you can look to the list of antediluvian kings in the Sumerian literature. The kings before the flood are called “sons of God.” Also on a tablet discovered at Ugarit is one called Krt who is said to be “a son of god” and “a king.” You see echoes of this use in the Scripture where rulers and judges are called “gods”: Psalm 138:1 (cf. verse 4); Psalm 82:6-7; Exodus 21:6; 22:8,9,28.
So this leaves us with two other equally good interpretations of this passage:
(1) that “sons of God” refers to the godly line of Seth (just mentioned in the chapter 5) who had sexual relations with the ungodly line of Cain (mentioned in chapter 4) and produced ungodly children.
(2) that “sons of God” refers to rulers (kings) who took whatever women they chose (married or not). This is an example of the sin of the time of Noah. Rulers abusing their power by means of violence (cf. Genesis 6:11,13).
The reason I don’t go with the angel interpretation is:
(1) The idea of angels marrying humans and producing giant offspring has no connection with the context. It doesn’t flow from what went before or what goes after. Where did the angels come from? What happened to them? How are they judged? Why are men destroyed for what the angels did?
(2) Christ Himself distinctly states that the angels cannot marry (Matthew 22:30).
(3) There is no reason to think that the Nephilim are the result of the union of the sons of God and the daughters of men. Please note that the Nephilim were already on the earth when the sons of God “went” (not “came down”) to the daughters of men (v. 4). The Nephilim were not a result of the union, since they were already present.
Concerning the Nephilim: Some people believe the Nephilim were a race of giants. They derive this from the mention of the Nephilim in Numbers 13:33 where it says, “And they spread among the Israelites a bad report about the land they had explored. They said, ‘The land we explored devours those living in it. All the people we saw there are of great size. We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them.’”
But if the Nephilim are a race of giants produced by the union of angels with women, how could the descendants of Anak come from the Nephilim? Wouldn’t the Nephilim have been destroyed in the flood along with all life (except for Noah’s family)? It seems obvious that something is confused about this interpretation.
Nephilim is a Hebrew word that probably comes from the word “to fall.” Therefore it means “ones who fall upon [others].” I follow Martin Luther in viewing the correct meaning as “tyrants” or vicious rulers. They are called Nephilim because they fell upon the people and oppressed them (cf. Genesis 6:11,13). If this is true, it doesn’t refer to a race of giants at all, but people with violent tendencies. (Giants aren’t mentioned anywhere in the book of Genesis.)
What about Numbers 13? Numbers 13 is the first mention of Anak. He seems to be the ancestor for a tribe of giants who lived along the southern part of Canaan and they held key cities like Hebron. During the time of conquest they were defeated and driven back to the Mediterranean coastline, but some of their descendants were still around in King David’s day (2 Samuel 21:15-22 - here called descendants of Rapha, but there is a Biblical connection between Rapha and Anak which is too hard to explain here). The giant Goliath was likely a descendant of Anak.
Fitting the Nephilim into Numbers 13:31-33, it would mean that the people of the land of Canaan were violent or their rulers were violent tyrants. The sons of Anak do seem to be large men (as the rest of the Scripture shows), but all Numbers would be saying is that the sons of Anak were violent men.
, then again, we knew that for some time now, didn’t we?