Thomas Aquinas

“ Just as the civic life denotes not the individual act of this or that one, but the things that concern the common action of the citizens, so the conjugal life is nothing else than a particular kind of companionship pertaining to that common action. ”

“ Matrimony is the bond by which they are tied formally, not effectively, and so it need not be distinct from the joining.
Reply to Objection 2: Although relation is not itself a sensible accident, its causes may be sensible. Nor is it necessary in a sacrament for that which is both reality and sacrament [*Cf. [4934] TP, Q [66] , A [1] ] to be sensible (for such is the relation of the aforesaid joining to this sacrament) , whereas the words expressive of consent, which are sacrament only and are the cause of that same joining, are sensible. ”

“ 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. ”

“ 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. ”

“ 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.
Art. 3 - Whether matrimony is fittingly defined in the text? Objection 1: It would seem that matrimony is unfittingly defined in the text* (Sent. iv, D, 27) . [*The definition alluded to is as follows: "Marriage is the marital union of man and woman involving living together in undivided partnership."] ”

“ It also indicates the force of this joining---for it is indissoluble---by the words "involving," etc.
The remaining definition indicates the effect to which matrimony is directed, namely the common life in family matters. And since every community is regulated by some law, the code according to which this community is directed, namely Divine and human law, finds a place in this definition. while other communities, such as those of traders or soldiers, are established by human law alone. ”