GENESIS 6 – WHO WERE "THE SONS OF GOD"?
GENESIS 6:1 Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, 2 that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose. 3 And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years." 4 There were giants 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. (NKJV)
There has been much speculation about who these "sons of God" mentioned in the 6th chapter of Genesis were. Three basic interpretations of this passage have been advanced.

The first, and oldest, belief is that "the sons of God" were fallen angels who consorted with human women, producing giant offspring called nephilim (Heb. рфмйн). This view was widely held in the world of the 1st century CE, and was supported by Flavius Josephus, Philo, Eusebius and many of the "Ante-Nicene Fathers," including Justin Martyr, Clement of Alexandria, Origen, Tertullian, Irenaeus, Athenagoras and Commodianus.

http://users.aristotle.net/~bhuie/gen6sons.htm