Well in the book of Job, it is generally agreed that the sons of god being referred to cannot be anything else but angels since they are described as existing some time before the creation of the heavens and the earth, singing in the presence of God himself as they witnessed the creation event.
We sing of the works of God's creation and yet we were not there.
Job 38:6 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof; 7 When the morning stars sang together, and all the sons of God shouted for joy? 8 Or who shut up the sea with doors, when it brake forth, as if it had issued out of the womb?
Since the earth was formed on day 3 and the stars on day 4, then we are not talking about the same day, now are we? So verse 7 is an inroad to Job as he was among the sons of God when they presented themselves 2 times in worship and praise for knowing better about God's hand in creation. Since verse 8 continues the series of question regarding God's work, I do not believe we should take verse 7 with verse 6 as meaning that was the when of it but when the stars sang together ( the time of their worship ) when the sons of God shouted for joy.
Job is the oldest book in the Bible, before even the first five books of Moses, and it gives a very strong indication of the known identity of the sons of god. Keeping the term "sons of god" in Genesis 6 in context, we see that neither Seth, his descendants or anyone else was ever referred to as "sons of god" in any kind of spiritual sense, nor in a physical sense minus Adam, since Adam, like the angels was a direct creation of God.
The English word sons is derived from the Hebrew word "ben" and it is defined in Strong's Concordance as..
HTML Bible Index - King James Version - Strongs Concordance - Frames Version
"from 'banah' (1129); a son (as a builder of the family name), in the widest sense (of literal and figurative relationship, including grandson, subject, nation, quality or condition, etc"
Since angels are not marrying nor given in marriage, ...
Matthew 22:29 Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God. 30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
....the sons of God as in the builder of the family name of Israel is the lineage of Seth.
John 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.
Since John 1:12-13 testify who anyone becomes the sons of God now by believing in Jesus Christ, then that which was the former mentioned was how they were identified as the sons of God before by bloodline and by Judaism. there is no point to refer to how it is not any more unless it was.
So when we are told that "men" began to multiply on the face of the earth, and that the sons of God saw the daughters of those "men" and took whoever they chose as wives, we see the human race, who are the generations of Adam clearly being distinguished from this other group referred to as the "sons of God". The clear cut logical meaning is that these beings were sons of God, rather than of men, or humanity. So in Genesis 6, you are only left with two groups, men and the daughters who were born from those men who are of the generations of Adam, and the other group called "the sons of God" who had to get a piece of all that action who are NOT of the generations of Adam and henceforth are not men by definition. So therefore, the sons of God can only be angels, unless you want to believe they were flying spaghetti monsters. Let reason prevail and the Bible will actually begin to make sense.
Yet the offspring were men, mighty men, men of renown, but still men. And it was not because of this mingling with angels which scripture says they cannot do, in being able to take women to be called "wives" to them in His words, but judgment came on mankind because of the wicked thoughts was continually. He said it before the flood and repeated it again after the flood.
Genesis 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. 3 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. 4 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. 5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. 6 And it repented the Lord that he had made man on the earth, and it grieved him at his heart. 7 And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. 8 But Noah found grace in the eyes of the Lord.
Genesis 8:20 And Noah builded an altar unto the Lord; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.
21 And the Lord smelled a sweet savour; and the Lord said in his heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart is evil from his youth; neither will I again smite any more every thing living, as I have done. 22 While the earth remaineth, seedtime and harvest, and cold and heat, and summer and winter, and day and night shall not cease.
Something to discern with the Lord about regarding these truths in His words.