Objection 1. It would seem that the woman should not have been made in the first production of things. For the Philosopher says (De Gener. ii, 3), that "the female is a misbegotten male." But nothing misbegotten or defective should have been in the first production of things. Therefore woman should not have been made at that first production.
Objection 2. Further, subjection and limitation were a result of sin, for to the woman was it said after sin (Genesis 3:16): "Thou shalt be under the man's power"; and Gregory says that, "Where there is no sin, there is no inequality." But woman is naturally of less strength and dignity than man; "for the agent is always more honorable than the patient," as Augustine says (Gen. ad lit. xii, 16). Therefore woman should not have been made in the first production of things before sin.
Objection 3. Further, occasions of sin should be cut off. But God foresaw that the woman would be an occasion of sin to man. Therefore He should not have made woman.
On the contrary, It is written (Genesis 2:18): "It is not good for man to be alone; let us make him a helper like to himself."
I answer that, It was necessary for woman to be made, as the Scripture says, as a "helper" to man; not, indeed, as a helpmate in other works, as some say, since man can be more efficiently helped by another man in other works; but as a helper in the work of generation. This can be made clear if we observe the mode of generation carried out in various living things. Some living things do not possess in themselves the power of generation, but are generated by some other specific agent, such as some plants and animals by the influence of the heavenly bodies, from some fitting matter and not from seed: others possess the active and passive generative power together; as we see in plants which are generated from seed; for the noblest vital function in plants is generation. Wherefore we observe that in these the active power of generation invariably accompanies the passive power. Among perfect animals the active power of generation belongs to the male sex, and the passive power to the female. And as among animals there is a vital operation nobler than generation, to which their life is principally directed; therefore the male sex is not found in continual union with the female in perfect animals, but only at the time of coition; so that we may consider that by this means the male and female are one, as in plants they are always united; although in some cases one of them preponderates, and in some the other. But man is yet further ordered to a still nobler vital action, and that is intellectual operation. Therefore there was greater reason for the distinction of these two forces in man; so that the female should be produced separately from the male; although they are carnally united for generation. Therefore directly after the formation of woman, it was said: "And they shall be two in one flesh" (Genesis 2:24).
Reply to Objection 1. 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, as the Philosopher observes (De Gener. Animal. iv, 2). 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.
Reply to Objection 2. Subjection is twofold. One is servile, by virtue of which a superior makes use of a subject for his own benefit; and this kind of subjection began after sin. 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. Nor is inequality among men excluded by the state of innocence, as we shall prove (96, 3).
Reply to Objection 3. If God had deprived the world of all those things which proved an occasion of sin, the universe would have been imperfect. Nor was it fitting for the common good to be destroyed in order that individual evil might be avoided; especially as God is so powerful that He can direct any evil to a good end.
Source: Summa Theologica (New Advent)