Aquinas’ views on female inferiority were doubtless influenced as well by Aristotle’s reproductive biology, with its understanding of the relation between male and female as one of active (perfect) principle to passive (imperfect) principle. Aristotle saw the sperm as the formative agent; the mother simply supplied raw material to be incorporated into the developing child. He also thought the sperm was directed to producing only male offspring, and that when this did not result it was because something interfered with the active principle within the sperm.
Finally, however, Aquinas does not believe it matters very much whether the particular causes involved in reproduction are to be regarded as failing or not failing when women are engendered. God desires that women be part of the universe, and He orders nature in such a way as to insure that they are produced. (On the question of Aquinas’ biology, see Michael Nolan, "What Aquinas Never Said About Women," FT, November 1998.)