Does Saint Thomas teach that femininity is a defect?

While Aquinas does use Aristotle in discussing man and woman, he explicitly rejects Aristotle's equating femininity with defect, or as some like to dramatically put it: "deformity". One of Aquinas’ theological arguments for this fact is that in the garden of Eden God created them male and female and the original state had no defect.

The famous phrase “femina est mas occasionatus” is sometimes translating by propagandists in a most derogatory and shocking way: "woman is a deformed man". This phrase does mean something like "woman is a malformed male"—or perhaps one could translate it as "occasioned male", "unintended male", "incomplete male" or various other renderings.
Femininity is indeed seen by Aristotle as in some sense a defect, but for Thomas this idea is explicitly abandoned. It is superficial and grossly misrepresentative to put that quote into Thomas' mouth and assume Aristotle's meaning. As indicated, for Thomas there is no ontological inferiority or defect in femininity. Man and woman are equal but distinct and that is it.

The major difference between Thomas and Aristotle in this regard is that for Aristotle femininity is a defect—women are “misbegotten”—but for Thomas femininity is in fact a perfection [tali perfectione, puta perfectione masculi, ille autem perfectione feminae] and a work of God’s wisdom and love in creation equal in dignity to that of masculinity. This is a radical difference and the people who propagate this tiresome slander against a great saint and doctor of the Church are not only being unfair, but are spreading sinister lies.

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 Theologiae, Prima Pars, Question 92, 1, ad 1.

Nothing belonging to the completeness of human nature would have been lacking in the state of innocence. And as different grades belong to the perfection of the universe, so also diversity of sex belongs to the perfection of human nature. Therefore in the state of innocence, both sexes would have been begotten. - Summa Theologiae, Prima Pars, Question 99, 2, ad 1.

Patet ergo quod de intentione agentis particularis est quod effectus suus fiat perfectus quantumcumque potest in genere suo: de intentione autem naturae universalis est quod hic effectus fiat perfectus tali perfectione, puta perfectione masculi, ille autem perfectione feminae. - Summa Contra Gentiles, 3, 94, n.10


As an aside—Bonaventure similarly rejects Aristotle's view in several places, for example: [In 2 Sent., 20, 6, ad 1] “generatio autem mulieris non est praeter naturam nec contra naturam sed secundum naturama.”
“The generation of a woman is neither outside Nature nor contrary to (contra) Nature, but according to (secundum) Nature.”

Aristotle pretty much denigrated anyone who was not an educated Athenian male, but this is not the Christian world-view. The authentic tradition affirms that man and woman are both created in the image of God and that this creation is very good. There is no defect in either sex, and the destiny of all human beings is the eternal enjoyment of Divine life, thus, all humanity, regardless of sex or race, has the same transcendent dignity and end. In Christ there is no male and female, Greek or Jew, et cetera.

Many of the cultural views, such as those regarding the subordination of women, are not of themselves statements about the dignity of woman much as the modern counterparts (such as that women are more emotionally mature, women are more loving and empathetic, etc.) are not necessarily derogatory toward men (although many radical feminists employ them in a way that denigrates masculinity).

References in Thomas:
In 2 Sent., 20, 2, 1, ad 1;
In 4 Sent., 44, 1, 3c, ad 3;
Summa Theologiae, 1, 92, 1, ad 1;
Summa Theologiae, 1, 99, 2, ad 1;
De Veritate, 5, 9, ad 9;
Summa Contra Gentiles, 3, 94, n.10.