The Sons of God Were the Sethites
There should be no confusion about who the sons of God are in Genesis, if you understand that there were two distinct peoples after the fall. Genesis says that Adam had sons and daughter's and after Cain killed Abel he wondered until he finally set up a city.
In order to found a city you have to have a multitude. Cain's first son was Enoch. Most do not realize there were two Enochs. When Adam was 130 he fathered Seth.
Seth was to pickup the line of righteous men followed by his listed descendents. Keep in mind that the world sunk into corruption quickly after the fall of Adam. Not all of Adam's sons and daughters were righteous. Perhaps many joined up with their older brother Cain in his city and continued down their path of corruption. Cain was never sorry for his actions.
The few Sethites (those righteous descendants of Seth) most likely avoided the rapidly growing and wicked Cainites. Here you have the two types of men. Those who follow God and those who do not. Those who are righteous are known as sons of God. Those who are wicked are known as sons of man. In this case the righteous men saw the daughters of the worldly and intermingled, or married them, ultimately turning the righteous men into wicked men.
You see this going on today. If you marry an ungodly, or an unbeliever, the result will ultimately be that you will begin to follow the world.
They had children, of course, and in those days there were giants. Man's stature, strength, longevity and mental capacity pales in comparison to those living before the flood. Within 400 years of the flood you see a rapid shortening of peoples lives.
Up until the tower of Babel there was one language and a great deal of the knowledge before the flood was known to Noah and his 3 sons and their wives. Much of this knowledge was passed down. When the languages were confounded at Babel, the mass of Noah's descendents splintered and moved off to form different countries.
They realized that the lives of their fathers and grandfathers were longer than their own, and they realized they could not retain the knowledge of their ancestors. So each group developed their own written language in an effort to retain some of this rapidly dwindling knowledge, thus they began to write things down.
The Book of Enoch was not written by either Enoch before the flood. Just because it has the name of someone does not mean that it was written by that person. The descendants of Noah also noticed that their stature and strength was diminishing as well, some faster than others. Noah and Seth remained alive all during this time even up to Abram's birth.
It is not at all unreasonable for either one of them to have met Abram, although this is not mentioned in the scriptures. Many of Ham, Shem and Japhath's son's and grandsons were giants as well, although maybe not as large as their fathers; but still much larger than the average person at Joseph's time. There were still some rather large stature persons at the time of David, such as Goliath.
While Goliath was probably not as tall as Shem or Ham, or their immediate descendants he would still seem like a giant to the Israelites in David's time and especially when the Israelites sent spies into the promised land. No doubt Abram was taller in stature and age as Jacob concedes to the Pharaoh of Egypt that his life is not as long and journeyed as his fathers and ancestors.
These giants, or Nephilim, were simply the mighty men of renown that lived long and had far superior knowledge before the flood and immediately afterward which began to depreciate rapidly. There are probably many reasons for the decline but God says he would shorten mens lives. When Noah came out of the ark, the entire earth was wrecked.
The soil was not as good in nutrients and the air pressure was most likely less and things just didn't grow as large. Noah and his family must have been very disheartened when the door opened and they saw the scarred and torn earth, knowing what beauty it was before the flood.
The book of Enoch is not divinely inspired. Many things in this book contradict the Word of God. The reference in Jude to Enoch is a reference to the man Enoch and not this questionable book of Enoch. Christ referred to Adam and yet there was no book of Adam. He was referring to the man Adam whom he had created. Those pushing the Book of Enoch are trying to introduce a false doctrine into God's word.
Claiming that there was a conspiracy to keep it out of the bible because it tells us what really happened back then. The way it is written, it's style and form are unknown from the Word of God. It introduces a new doctrine foreign to Salvation through Jesus Christ. Everything God has done, Satan always has a counterfeit.
When the book of Enoch is compared to Gods Word "To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.” Isaiah 8:20 the Book of Enoch fails miserably.