The greatest of human friendships

"if a husband were permitted to abandon his wife, the society of husband and wife would not be an association of equals, but, instead, a sort of slavery on the part of the wife [(non esset aequa societas viri ad mulierem, sed servitus quaedam ex parte mulieris)]."

"the greater the friendship is, the more solid and long-lasting will it be.  Now, there seems to be the greatest friendship [(maxima amicitia)] between husband and wife, for they are united [(adunatur)] not only in the act of fleshly union [(in actu carnalis copulae)], which produces a certain gentle association [(quandam suavem societatem)] even among beasts, but also in the partnership of the whole range of domestic activity [(ad totius domesticae conversationis consortium)].  Consequently, as an indication of this, man must even 'leave his father and mother' for the sake of his wife, as is said in Genesis (2:24).  Therefore, it is fitting for matrimony to be completely indissoluble [(omnino indissolubile)]."

     St. Thomas Aquinas, ScG (revised and completed 1260-65) III.123.4 & 6, trans. Vernon J. Bourke.  Latin from Corpus Thomisticum.  Note that ad by comparison with that in, both of which qualify adunatur:  "in the act of fleshly union," but "towards" or "with an eye to" or "for" partnership in the whole of domestic association.
     The sacramentality of marriage:  But then note that friendship (amicitia)—which is realized first and preeminently within the life of the triune God himself (Dictionnaire de philosophie et de théologie Thomistes, 18)—is, for Aquinas, precisely what the heavenly Bridegroom seeks with us.

"the perfect friendship of the sort which exists between a man and his wife [(perfecta amicitia qualis est inter virum et uxorum)], for whom man even leaves his father and mother (Gen 2:24), cannot be had with many wives."

     St. Thomas Aquinas, Expositio super Isaiam ad litteram (1251/52, i.e. before the Commentary on the Sentences) 4.1.136, trans. Robert St. Hilaire.  "what, for Aquinas, is [truly] essential to marriage, the communion of life between the spouses and [their] perfect friendship, is the sole true reason for avoiding polygamy.  If, three years later, in the commentary on the Sentences, he evokes also the good of children and mutual aid, this is because he expresses himself [there] in a manner more complete.  [But] the early commentary on Isaiah manifests already very clearly the conception of the essence of marriage that we encounter [again and again] in the course of his teaching" later on (Adriano Oliva, O.P., "Essence et finalité du marriage selon Thomas d'Aquin pour un soin pastoral renouvelé," Revue des sciences philosophiques et théologiques 98, no. 4 (2014):  611 (601-668), translation mine).
     Against the argument that this conception of the true essence (or first—as distinguished from consequent—perfection) of marriage renders it vulnerable to the reformulations of our time would have to stand, among other things, the fact that "male and female have different operations," and that "the vice of sodomy," as "entirely unnatural," can "in no manner stand with the stated end" (135 and both often elsewhere in the Thomistic corpus).  More important still:  "things that are ordered to some one thing are said to be united together in their ordering to it [(in ordine ad aliud)]. . . .  And so, since by marriage two people are ordered to [1] one single generation and education of children, and also to [2] one single domestic life, it is clear that in marriage there is a union, because of which a man and a woman are called 'husband' and 'wife'; and such a union, by the fact that it is ordained to some one thing [(ex hoc quod ordinatur ad aliquod unum, most distinctively that 'one single generation . . . of children')] is marriage" (In IV Sent. 27.1.1.1.Resp., italics mine).