Reply to Objection 6: As Augustine says (De Civ. Dei xv): "Many persons affirm that they have had the 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." 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; just as they take the seed of other things for other generating purposes, as Augustine says (De Trin. iii), so that the person born is not the child of a demon, but of a man.
http://www.ccel.org/ccel/aquinas/summa.FP_Q51_A3.html
The intercourse between humans and demons that St.Augustine is referring to is a spiritual, not necessarily physical, intercourse which is something common among pagan sorcerers in many of their rituals. What the devil is doing is engaging the shaman (or whatever) in a kind of sɛҳuąƖ ecstasy which occultists consider a kind of religious experience often accompanying their attainment of "enlightenment".
For witches this is followed by intercourse with a human male (sometimes possessed). The result is a man child possessed by a devil when it is born because it has been consecrated to it in the womb through this ritual.
While I havent found any old accounts of demon possessed people commiting rapes, I do know of quite a few modern day accounts where someone was demonically possessed and went on to commit a rape. I do not recall reading of any children being born of the female victims, but I think we can assume that the child born of such an intercourse can become a kind of Hercules of the devil, which is what the Giants mentioned in Genesis seem to be, considering that many ancient pagan heroes of strength and power (among the Celts especially) share many of their attributes. If they existed before and after the flood, I think we can assume that the super-soldiers of satan are still being produced today.
It's just a complicated possession, not creation.