From "Five Fusty Excerpts from Archaic 'Nonfiction' Texts" by Julia Elliott, in Electric Lit:
Aquinas also gives us the dirt on angelic sin and demons. In this English translation of “Reply to Objection 6” or “Whether the angels exercise functions of life in the bodies assumed,” he answers the burning question: can an incubus knock up a mortal woman?
"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 . . . , so that the person born is not the child of a demon, but of a man."