Objection 1. It would seem that matrimony is not a kind of joining. Because the bond whereby things are tied together differs from their joining, as cause from effect. Now matrimony is the bond whereby those who are joined in matrimony are tied together. Therefore it is not a kind of joining.
Objection 2. Further, every sacrament is a sensible sign. But no relation is a sensible accident. Therefore since matrimony is a sacrament, it is not a kind of relation, and consequently neither is it a kind of joining.
Objection 3. Further, a joining is a relation of equiparance as well as of equality. Now according to Avicenna the relation of equality is not identically the same in each extreme. Neither therefore is there an identically same joining; and consequently if matrimony is a kind of joining, there is not only one matrimony between man and wife.
On the contrary, It is by relation that things are related to one another. Now by matrimony certain things are related to one another; for the husband is the wife's husband, and the wife is the husband's wife. Therefore matrimony is a kind of relation, nor is it other than a joining.
Further, the union of two things into one can result only from their being joined. Now such is the effect of matrimony (Genesis 2:24): "They shall be two in one flesh." Therefore matrimony is a kind of joining.
I answer that, A joining denotes a kind of uniting, and so wherever things are united there must be a joining. Now things directed to one purpose are said to be united in their direction thereto, thus many men are united in following one military calling or in pursuing one business, in relation to which they are called fellow-soldiers or business partners. Hence, since by marriage certain persons are directed to one begetting and upbringing of children, and again to one family life, it is clear that in matrimony there is a joining in respect of which we speak of husband and wife; and this joining, through being directed to some one thing, is matrimony; while the joining together of bodies and minds is a result of matrimony.
Reply to Objection 1. 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. III, 66, 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.
Reply to Objection 3. A relation is founded on something as its cause--for instance likeness is founded on quality--and on something as its subject--for instance in the things themselves that are like; and on either hand we may find unity and diversity of relation. Since then it is not the same identical quality that conduces to likeness, but the same specific quality in each of the like subjects, and since, moreover, the subjects of likeness are two in number, and the same applies to equality, it follows that both equality and likeness are in every way numerically distinct in either of the like or equal subjects. But the relations of matrimony, on the one hand, have unity in both extremes, namely on the part of the cause, since it is directed to the one identical begetting; whereas on the part of the subject there is numerical diversity. The fact of this relation having a diversity of subjects is signified by the terms "husband" and "wife," while its unity is denoted by its being called matrimony.
Source: Summa Theologica (New Advent)