Thomas Aquinas, answered from the texts and cited to the page.
The question admits of several distinct levels, and collapsing them produces the confusion that most modern readers bring to the texts. Begin with what I said about woman considered as an individual product of generation. Following the Philosopher, I wrote that as regards the individual nature, woman is defective and misbegotten, for the active force in the male seed tends to the production of a perfect likeness in the masculine sex, and the production of woman comes from some defect in the active force, from material indisposition, or from external influence such as a moist south wind.1
That is a claim in natural philosophy, borrowed from Aristotle's De Generatione Animalium, and it concerns the proximate efficient cause of sexual differentiation, not the worth of women as persons or their standing before God. The qualification follows immediately and matters equally. As regards human nature in general, woman is not misbegotten but is included in nature's intention as directed to the work of generation, and since the general intention of nature depends on God who is the universal Author of nature, God formed not only the male but also the female.2
Diversity of sex belongs to the perfection of human nature, just as different grades of being belong to the perfection of the universe.3 The state of innocence would have included both sexes, and males and females would have been born in equal number.4 Woman is called misbegotten only relative to the particular intention of the generative act, not relative to the order of the whole.
On subjection: I distinguished two kinds carefully. Servile subjection, by which a superior uses a subject for his own benefit, began after sin and was no part of the original order. Economic or civil subjection, by which a superior governs subjects for their own benefit and good, existed even before sin, because good order would have been wanting in the human family if some were not governed by others wiser than themselves.5
Woman is naturally subject to man in this second sense, because in man the discretion of reason predominates — which is a claim about governance within the household, not a claim that women lack reason or are incapable of virtue. The commentary on Aristotle's Politics presses the same distinction further. Woman has a natural disposition to beget from another but is not robust in body in the way required of a slave, so the two associations — woman with man, slave with master — differ from each other in kind.6
The point is that woman is not a slave; the relation is ordered to generation and to the common life of the household, not to the master's private advantage. On the soul there is no subordination at all. The rational soul exceeds the whole nature and power of the body and is brought into existence not by any bodily power in the semen but by God alone through creation.7
Since the soul is the same in kind for man and woman — both created immediately by God, both subsistent, both ordered to beatitude — the subjection I describe is entirely in the order of nature considered as a bodily and social arrangement, not in the order of intellect or grace. What is genuinely in tension across these positions is the borrowed Aristotelian biology and the theological commitments that surround it.
The biology treats femaleness as a departure from the primary intention of the generative power; the theology treats both sexes as equally intended by God, equally ensouled by divine creation, and equally ordered to the same last end. I held both, and a serious reader must hold both in view rather than selecting one to condemn the other.
As regards the individual nature, woman is defective and misbegotten, for the active force in the male seed tends to the production of a perfect likeness in the masculine sex; while the production of woman comes from defect in the active force or from some material indisposition, or even from some external influence; such as that of a south wind, which is moist.Summa Theologica Part I, Prima Pars
On the other hand, as regards human nature in general, woman is not misbegotten, but is included in nature's intention as directed to the work of generation. Now the general intention of nature depends on God, Who is the universal Author of nature. Therefore, in producing nature, God formed not only the male but also the female.Summa Theologica Part I, Prima Pars
And as different grades belong to the perfection of the universe, so also diversity of sex belongs to the perfection of human nature.Summa Theologica Part I, Prima Pars
From this it seems to follow that males and females would have been in equal number.Summa Theologica Part I, Prima Pars
There is another kind of subjection which is called economic or civil, whereby the superior makes use of his subjects for their own benefit and good; and this kind of subjection existed even before sin. For good order would have been wanting in the human family if some were not governed by others wiser than themselves. So by such a kind of subjection woman is naturally subject to man, because in man the discretion of reason predominates.Summa Theologica Part I, Prima Pars
For a woman has a natural disposition to beget from another, but she is not robust in body, which is what is required in a slave. And so the two associations just mentioned differ from each other.Commentary on Aristotle's Politics, Book I and Book III partial, p. 7
The rational soul exceeds the whole nature and power of the body, since no body can rise to the heights of the soul's intellectual activity... the power possessed by a body cannot produce the rational soul, nor, consequently, can the energy inherent in the semen do so.Compendium of Theology, pp. 60–61