What are the sons of God?  According to the Bible, there are only two distinctions in the Bible.  Those whom obey the commandments of God, and those whom do not.  So what was Genesis 6:4 speaking of then?  To some, the word Nephilim itself has been attributed to the term "the fallen ones", and in this same sense, they attribute it to the fallen angels.  The fallen ones per the scripture, however, are not those angels that initially deceived the world, but those whom God created who decided to turn against him and defy his divine authority.  Although the 10 commandments were not given to Israel at this point, Eve had still been deceived by Azazel, Cain still went up against Abel, and there was sin in the world.  Men, living in their sin, conceived daughters, and based on their appearance, the sons of God (those whom were like Abel) were led astray from the truth defying the will of God, being wicked in his eyes.  Based on this, God saw favor in Noah, advised him to create an ark, and destroyed the population of the world save those whom went into the Ark.