Objection 1. It would seem that matrimony is unfittingly named. Because a thing should be named after that which ranks higher. But the father ranks above the mother. Therefore the union of father and mother should rather be named after the father.
Objection 2. Further, a thing should be named from that which is essential to it, since a "definition expresses the nature signified by a name" (Metaph. iv, 28). Now nuptials are not essential to matrimony. Therefore matrimony should not be called nuptials.
Objection 3. Further, a species cannot take its proper name from that which belongs to the genus. Now a joining [conjunctio] is the genus of matrimony. Therefore it should not be called a conjugal union.
On the contrary, stands the common use of speech.
I answer that, Three things may be considered in matrimony. First, its essence, which is a joining together, and in reference to this it is called the "conjugal union"; secondly, its cause, which is the wedding, and in reference to this it is called the "nuptial union" from "nubo" [The original meaning of 'nubo' is 'to veil'], because at the wedding ceremony, whereby the marriage is completed, the heads of those who are wedded are covered with a veil [This is still done in some countries]; thirdly, the effect, which is the offspring, and in reference to this it is called "matrimony," as Augustine says (Contra Faust. xix, 26), because "a woman's sole purpose in marrying should be motherhood." Matrimony may also be resolved into "matris munium" [i.e. munus], i.e. a mother's duty, since the duty of bringing up the children chiefly devolves on the women; or into "matrem muniens," because it provides the mother with a protector and support in the person of her husband; or into "matrem monens," as admonishing her not to leave her husband and take up with another man; or into "materia unius," because it is a joining together for the purpose of providing the matter of one offspring as though it were derived from monos and "materia"; or into "matre" and "nato," as Isidore says (Etym. ix), because it makes a woman the mother of a child.
Reply to Objection 1. Although the father ranks above the mother, the mother has more to do with the offspring than the father has. or we may say that woman was made chiefly in order to be man's helpmate in relation to the offspring, whereas the man was not made for this purpose. Wherefore the mother has a closer relation to the nature of marriage than the father has.
Reply to Objection 2. Sometimes essentials are known by accidentals, wherefore some things can be named even after their accidentals, since a name is given to a thing for the purpose that it may become known.
Reply to Objection 3. Sometimes a species is named after something pertaining to the genus on account of an imperfection in the species, when namely it has the generic nature completely, yet adds nothing pertaining to dignity; thus the accidental property retains the name of property, which is common to it and to the definition. Sometimes, however, it is on account of a perfection, when we find the generic nature completely in one species and not in another; thus animal is named from soul [anima], and this belongs to an animate body, which is the genus of animal; yet animation is not found perfectly in those animate beings that are not animals. It is thus with the case in point. for the joining of husband and wife by matrimony is the greatest of all joinings, since it is a joining of soul and body, wherefore it is called a "conjugal" union.
Source: Summa Theologica (New Advent)