Augustine and Aquinas on "Sons of God" in Genesis 6

Answering the question of whether Genesis 6:1-2 indicates intermarriage of demons with humans, Thomas Aquinas writes in Summa Theologiae, Part I, Question 51, Article 3:

As Augustine says (De Civ. Dei xv): "Many persons affirm that they have had experience, or have heard from such as have experienced it, that the Satyrs and Fauns, whom the common folk call incubi, have often presented themselves before women, and have sought and procured intercourse with them. Hence it is folly to deny it. But God's holy angels could not fall in such fashion before the deluge. Hence by 'the sons of God' are to be understood the sons of Seth, who were good, while by the daughters of men the Scripture designates those who sprang from the race of Cain. Nor is it to be wondered at that giants should be born of them, for they were not all giants, albeit there were many more before than after the deluge.

Thomas continues:

Still, if some are occasionally begotten from demons, it is not from the seed of such demons, nor from their assumed bodies, but from the seed of men taken for the purpose, as when the demon assumes first the form of a woman, and afterwards of a man...so that the person born is not the child of a demon, but of a man.