Dates: c. 373 Summary: Ephrem the Syrian (d. 373) holds a purely Sethite reading. The "sons of God" are the righteous descendants of Seth and the "daughters of men" are the adorned daughters of Cain who ensnared them. Ephrem further explains the giants' physical strength naturally: the Sethites were strong because they dwelt in the blessed land near paradise, while the cursed Cainites grew weak on land that withheld its strength.
He called the sons of Seth sons of God, those who, like the sons of Seth, had been called "the righteous people of God." The beautiful daughters of men whom they saw were the daughters of Cain who adorned themselves and became a snare to the eyes of the sons of Seth. Then Moses said, they took them wives of all which they chose.
— Commentary on Genesis, pages 134-135, source. Ephrem reads the union as intermarriage between the two human lines, not angelic descent.
The house of Cain, because the earth had been cursed so as not to give them its strength, produced small harvests, deprived of its strength, just as it is today that some seeds, fruits and grasses give strength and some do not. Because at that time they were cursed and sons of the cursed and were dwelling in the land of curses, they would gather and eat produce that lacked nutrition, and those who ate these were without strength just like the food that they ate. As for the Sethites, on the other hand, because they were the descendants of the blessed [Seth] and were dwelling in the land along the boundary of the fence of paradise, their produce was abundant and full of strength. So too were the bodies of those who ate that produce strong and powerful.
— Commentary on Genesis 6.5.1, in St. Ephrem the Syrian: Selected Prose Works (Fathers of the Church 91; trans. E. G. Mathews & J. P. Amar, 1994), Google Books. Ephrem gives a wholly natural account of the giants' might, grounded in geography and diet rather than in any superhuman parentage.