Genesis 6:1-4, “When men began to increase in number on the earth and daughters were born to them, the SONS OF GOD saw that the daughters of men were beautiful and they married any of them they chose. Then the LORD said, “My Spirit will not contend with man forever, for he is mortal; his days will be an hundred and twenty years”. The Nephilim were on the earth in those days- and also afterward- when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.” (NIV)
This is a knotty theological issue that has remained inconclusive for years. I don’t want to present my answer to be the only right answer in this matter because we are still learning but because we know in part, we can only interpret verses such as these as we deem fit and I align with what I can prove from the scriptures - not just some religious myth. 
Now, there are three major theological perspectives to this issue. 
Some scholars hold the view that the phrase ‘Sons of God’ refers to fallen angels and that those angels slept with the daughters of men to give birth to NEPHILIMS referring to extraordinary men in physique-  believed to be giants.
Another school of thought holds the view that the ‘Sons of God’ were influential men of those days who were referred to as ‘Sons of god’ because they were powerful. This has some connection with idolatrous worship of certain gods in the east, hence, adherents referred to them as ‘sons of god’.
Another scholarly view is that the ‘Sons of God’ referred to the godly lineage of Seth and Enosh because Genesis 4:26 says “Seth also had a son, and he named him Enosh. At that time men began to call on the name of the LORD. “So we can infer that there was a generation of ‘godly’ men right from the time Enosh was given birth to until when the flood came.
THEORY ONE DEBUNKED
Personally, I do not agree that it was angels that were referred to as Sons of God in Genesis6, neither do I agree that angels married the daughters of men and slept with them to produce offspring. Most of the people that hold this view refer to Job1:6, 2:1 and 38:7 as proof that the Sons of God referred to angels.
Job 1:6, “Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.”
Job 2:1,“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.”
When you look at these scriptures critically, truly the sons of God referred to here were angels because the place of meeting was not on this earth probably in the heavenly court. Satan wouldn’t have appeared amongst humans in their place of worship. However, there are more than three places in the scriptures where human beings were also referred to as ‘Sons of God’.
Hosea 1:10, “Yet the number of the Children of Israel shall be as the sand of the sea…there it shall be said unto them, Ye are the sons of the living God.”
Hosea 11:1, “When Israel was a child, then I loved him, and called my son out of Egypt.”
Deuteronomy 14:1 “Ye are the children of the LORD your God:…”
Isaiah 43:6,“I will say to the north, Give up; and to the south, keep not back: bring my sons from far, and my daughters from the ends of the earth;”
Psalms 82:6, “I said, ‘Ye are gods; and all of you are Children of the most High”.
When you look at these scriptures put together, they are referring to humans.
So humans were referred to as either ‘Sons of God’, ‘Children of the most High’, etc. So one cannot hold a rigid view as regards Genesis 6 as strictly referring to angels because of Job 1:6,2:1 and 38:7. One would have to look at other verses of the scriptures where such expressions were used. You need to understand that the bible was not explicit as to whether Genesis 6 referred to angels, else we may be giving the scripture a private interpretation and relying on religious myths handed down. We also need to know what Jesus said about angels in Matthew 22:30, “For in resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.”
What can we infer from this scripture?
(A) Angels do not marry whereas  the ‘Sons of God’ in Genesis 6 married, so they wouldn’t have been angels.
(B) If angels do not marry, then they cannot have sexual intercourse with men. In fact, when men of Sodom and Gomorrah wanted to violate this order of angelic creation, the angels struck them with blindness in Genesis19:10-11. So from scriptures, we see that angels can neither marry nor have sex.
In fact, angels do not have a specific gender though the masculine gender favours them in writing but bible grammar mostly refers to male and female with a general terminology ‘men’. That doesn’t make the women to become men, it is just the bible way of writing.
THEORY ‘2’ DEBUNKED
It is clear that the scripture used capital ‘G’ for God not ‘gods’ as some scholars opined; hence in Genesis 6, the Hebrew rendition is ‘Elohiym’ referring to God and not an idol. So that knocks out the argument that opines that the ‘Sons of God’ were some influential or powerful men with some idolatrous connection to a god.
Again, we must not forget that the event recorded in Genesis 6:1-4 happened before the flood and prior to that time, God didn’t give any specific command against men intermarrying so why would God be angry with that generation that would make him go as far destroying them all except Noah and his family? Though, those that hold the first view believed that it must be because of the bizarre relationship between angels and daughters of men which led to the birth of giants. I still do not agree with that view because of the plethora of scriptural evidences against that view. Giants existed before and after the flood. (Number13:33). If the sexual union between angels and daughters of men produced giants before the flood that swept the earth clean, what about the giants that surfaced after the flood, how were they produced? At least, you can remember Goliath in 1Samuel 17.
THEORY 3 EXPLAINED
Of all the theories or scholarly perspectives, I believe the third theory has biblical proofs and is consistent with the order of creation. Now let’s explain it in bits.
(A) Since we’ve seen from the scriptures that the phrase ‘Sons of God’ could refer to angels and humans as well, it is plausible that it is the ‘godly’ generation of Seth that was being referred to in Genesis 6 because angels don’t marry and cannot have sexual intercourse as explained in Matthew22:30, Mark12:25, Luke20:35, 36.
(B) The bible told us in Genesis 4:26 that “Seth also had a son, and he named him Enosh. At that time, MEN BEGAN TO CALL ON THE NAME OF THE LORD. “So we can infer that there was a generation of ‘godly’ men right from the time Enosh was given birth to until when the flood came.
TWO GENEALOGIES COMPARED
In Genesis 4, Cain killed Abel and we saw from scriptures that he was a wicked man. He was godless and ruthless. His sin brought a curse upon him in vs11 -12. In verse17-22, the genealogy of Cain was shown to us in brief and we saw that his generation produced the first polygamist (which was against the original plan of God for marriage)- Lamech.
See what Lamech said in vs 23-24, “Lamech said to his wives,  ‘Adah and Zillah, listen to me; wives of Lamech, hear my words. I HAVE KILLED A MAN FOR INJURING ME. If Cain is avenged seven times, then Lamech seventy-seven times.’”
That sounds like a wicked man and truly he was a wicked man because he was a murderer and he was proud of it. The book of Psalm7:11 made it clear that God is angry with the wicked every day. There is a high tendency that he would also raise wicked, godless sons and daughters. I believe this wicked generation produced the daughters of men that were referred to in Genesis 6.
In Genesis 5, another generation was shown us and we can point to notable men in that lineage that were outstanding for God. In vs24, we were told that Enoch walked with God and he was not for God took him. That is a godly man in the lineage of Seth. Another notable man in that lineage was Noah in vs29. So we can see that the generation of Seth had godly men that were notable. Hence, there is a high tendency that they were the ones referred to as ‘Sons of God’ while the generation of Cain were the daughters of men that they slept with. They became corrupted in their marriage relationship and we know that in the Old Testament, God forbade the intermarriage of the godly and the ungodly which is corroborated in 2Corinthians 6:14. Hence we can presume that it was the reason for the multiplication of sin before the flood because godless mothers would raise godless children and that necessitated the destruction of that generation with the flood. In Genesis 6, it was clear in vs3-7, 12 and 13 that God punished the sins of humans not angels. If angels were the ‘Sons of God’ intended in this chapter, the bible would have mentioned it. However, those verses above showed that God’s anger and punishment was meted out to humans.
Conclusively, I am not trying to give an authoritative view on this matter as we all know in part and we prophesy in part, but when that which is perfect is come, then that which is in part shall be done away with.1 Corinthians 13:9-10.