Let's look at the Scripture passage in question, in context:
Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, "My Spirit shall not strive with man forever, because he also is flesh; nevertheless his days shall be one hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown. Then the LORD saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually. (Genesis 6:1-5)Since the Bible indicates that angels are asexual beings, it makes sense that they could not be the "sons of God" who produced children with the "daughters of men." The best interpretation is that the "sons of God" were men who were descended from Seth, who followed the Lord for a time (in contrast to the line of Cain, which produced the "daughters of men"). However, right before the flood, even the "sons of God" took wives among the line of Cain, and, therefore, became corrupted themselves through their unbelieving wives. This is one of the reasons God determined to destroy the entire human race, except for the eight people who still followed the Lord (Noah and his extended family).
Genesis 6 also describes the Nephilim, who were the corrupt strongmen of their time, notorious for their violent exploits (Genesis 6:4). These men were probably also descendents of Cain, who were terrorizing the peoples and represented at least part of the group whose thoughts were "only evil continually." The Nephilim that were described after the flood were also evil strongmen, but not related to those pre-flood people, since they were all destroyed in the flood.
(For historical fiction that touches on this topic, see Chapter 1: Lion Cub of The Coming Wrath)
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(extracted from Who Were the Nephilim and Sons of God?, by Rich Deem, GodAndScience.org)