Fr. Seraphim says that the Patristic teaching is that the sons of God were the offspring of Seth, hte chosen ppl meant to prepare themselves in virtue, and the daughters of men are the offspring of Cain -- the forbidden outcasts. He mentions St. Ephraim the Syrian as an example.
regarding hte giants, Fr. Seraphim writes "By 'giants' here we do not need to understand enormous men. According to St. Ephraim, the offspring of Seth, the chosen race, were tall and full in stature, while hte offspring of Cain, the cursed one, were small. When these two races mixed, the tallness of the Sethites prevailed. The 'giant' stature of the men -- the descendants of Seth -- before the Flood is apparently one of the attributes of humanity that was lost with the new climactic conditions of the post-Flood world."