And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
 And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
There is some dispute over who the "sons of God" are in this passage. I believe it is referring to angels who married daughters of men and that this union produced giants.
A popular alternative view is that the sons of God were saved people. I see several problems with this view.
- If the sons of God here are saved people, what was the big deal with them getting married? That is not sin, so why does the text portray them getting married as a bad thing?
- Saved people getting married does not produce giants. But the text says "There were giants in the earth in those days...when the sons of God came in unto the daughters of men, and they bare children to them".
- Saved people in the Old Testament were not called "sons of God". Israel as a nation was God's son (Exod 4:22, Deut. 14:1-2, 32:18, Isa. 1:2, 44:1-4, 46:3, 66:8, Jer. 31:9, Hosea 1:10, 11:1, etc.). But individuals could not become spiritual sons of God until they received Christ and the Holy Spirit (John 1:11-12, 7:38-39, Rom. 8:14-16, Gal. 4:5-6).
- Some people suggest that the "sons of God" in the passage are the sons of Seth (a "godly line") and that they sinned by marrying the daughters of Cain (an "ungodly line"). However, there is no scripture that teaches this whatsoever. Besides that, such marriages would not produce giants.
I believe that Genesis 6 is referring to angels. Look at these other references to "sons of God"
 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
 And the LORD said unto Satan, Whence comest thou? Then Satan answered the LORD, and said, From going to and fro in the earth, and from walking up and down in it.
 Again there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them to present himself before the LORD.
 And the LORD said unto Satan, From whence comest thou? And Satan answered the LORD, and said, From going to and fro in the earth, and from walking up and down in it.
These sons of God "CAME" to present themselves "BEFORE THE LORD". Humans cannot just go up to Heaven to present themselves before the Lord. This would have to be referring to angels. [And it is very clear that they went up to Heaven, because the Lord asked Satan where he came from and Satan's answer was "from...the Earth". So this angelic meeting is not taking place on the Earth.]
Also notice that the "sons of God" were in existence before man was created on day six.
 Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
 Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
 When the morning stars sang together, and all the sons of God shouted for joy?
It is clear that the sons of God were present when God laid the foundation of the Earth, which was done on day 3 of the creation week (Gen. 1:9-10, Prov. 8:26-29). But man was not created until day 6 of the creation week (Gen. 1:26-31). Therefore, the sons of God in the Old Testament were definitely not people. [Note: the "stars" in Job 38:7 are a reference to angels (Rev. 1:20, 9:1)]
This does cause us to wonder, why are angels called "sons of God"? I believe it is because they were directly created by God. Notice that Adam is called a son of God because he was a direct creation
Luke 3:38 Which was the son of Enos, which was the son of Seth, which was the son of Adam, which was the son of God.
Adam was the son of God because he was directly created by God.
Psalms 82 is a very interesting passage, one that refers to the angels as "gods" and "children of the most high". In particular it refers to God judging them at the flood shortly after Genesis 6.
 God standeth in the congregation of the mighty; he judgeth among the gods.
 How long will ye judge unjustly, and accept the persons of the wicked? Selah.
 Defend the poor and fatherless: do justice to the afflicted and needy.
 Deliver the poor and needy: rid them out of the hand of the wicked.
 They know not, neither will they understand; they walk on in darkness: all the foundations of the earth are out of course. [reference to the flood, Job 9:5-6, 12:15, 22:16, 2 Pet. 3:6]
 I have said, Ye are gods; and all of you are children of the most High.
 But ye shall die like men, and fall like one of the princes.
 Arise, O God, judge the earth: for thou shalt inherit all nations.
The angels in Genesis 6 ("gods", "sons of God") were judged by God during the flood and they died like men.
One objection people have to this view is that they say angels cannot marry human women. They use the following two passages to teach this:
Matthew 22:30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
Mark 12:25 For when they shall rise from the dead, they neither marry, nor are given in marriage; but are as the angels which are in heaven.
However, those verses do not say that angels cannot come to Earth and marry human women. In fact, it says the exact opposite. It says the angels which are "IN HEAVEN" do not marry. Angels are always MEN in the Bible (Gen. 18:1-2, Acts 10:3-4, Rev. 10:1, 19:10, 22:8-9, Michael, Gabriel, etc.) so of course they do not marry "in heaven". That isn't even the issue. This is just a straw man argument. When we are talking about Genesis 6 we are talking about "the angels which kept not their first estate, but left their own habitation" (Jude 6). We aren't talking about the angels in Heaven.
Jude 6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
The offspring of angels and humans would certainly be unique, it is understandable that they would produce giants. See these descriptions of giants in the Bible: 1 Sam. 17:4-7, Numbers 13:33; Deuteronomy 2:11, 20, 3:11-13; Joshua 12:4, 13:12, 15:8, 17:12; 2 Samuel 21:16, 18, 20, 22; 1 Chronicles 20:4, 6, 8; Job 16:14.
One final objection people have to the sons of God being angels is that it would contradict Hebrews 1:5
"For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?"
But again, that isn't even the issue. That is a straw man argument. I am not saying that any of the angels are THE Son of God, capital s. I am simply pointing out that the Bible calls people who were directly created by God sons of God, such as Adam (Luke 3:38). God referred to the angels as "gods" (lower case g) and "children of the most High" in Psalms 82. But of course none of the angels are like unto Jesus Christ.
Pastor David O'Steen of Hope Bible Church has notes on this topic here:
--Eli "Hoss" Caldwell