Whatever we may think of that, the feast of our Lady’s Motherhood reminds us of a truth of outstanding importance in this time of confusion about marriage and family: our Lady was not, as some ignorant preachers are fond of saying, an “unwed mother.” When expecting the child Jesus, she was betrothed to St. Joseph, which, in the Jewish context, gave her the legal right to be regarded and treated as his spouse. Her wifehood, in other words, was as real as the foster fatherhood of Joseph or the adoptive relationship of so many saints of the Old Covenant; she could not, with any justice, be referred to as a mere fiancée. (Cardinal Burke has utterly shredded the view of Mary as “having a baby out of wedlock”; see his talk here.)
But this truth must necessarily prompt another question: How can we say that Mary and Joseph were ever married at any point in their lives? Is not the consummation of a marriage part of its very definition? In fact, recently a couple of college students posed this very question to me, and suggested that maybe Mary and Joseph were “just friends” all their lives, engaged, but never married. I said: “No, that’s not right; the tradition of the Church (as we can see in the liturgy) always refers to Joseph as ‘the husband of Mary’ and to the three as ‘the Holy Family’ and always holds up Mary and Joseph as exemplars for spouses.”
But the argument from authority, although sufficient, can be unsatisfying, so here is a proper explanation (which, by the way, I also gave to the college students).
According to St. Thomas, Mary and Joseph “consented to the conjugal uniting, but not expressly to the fleshly uniting save on condition that it should please God”; hence their marriage really came into existence. Thomas concludes his discussion with a quotation from Augustine:
All the goods of marriage are fulfilled in these parents of Christ: offspring, fidelity, and sacrament. The offspring we know to have been the Lord Jesus himself; fidelity, because there was no adultery; sacrament, because there was no divorce. Only marital intercourse was not present there.A modern reader is tempted to say: “Only? If this is lacking, how can the rest of it be a true marriage?” Indeed, Aquinas himself raises the same objection in this context: “Where the final consummation is lacking, there is no true completion.”
St. Thomas can argue the way he does because, in his view, the consent to marriage is not a consent to carnal intercourse, but a consent to the marital consortium or societas that implies such intercourse, or put differently, a consent to the mutual power of the spouses over each other’s bodies that is explicitated in the commixtio carnalis:
As fleshly uniting stands related to marriage, so likewise the consent that causes marriage stands related to consent to fleshly uniting. Now marriage, as was said above, is not essentially the fleshly conjoining itself, but a certain association of a man and a woman with a view to fleshly uniting and the other things that pertain, as a consequence, to the husband and wife, according as power is mutually given to them with respect to the fleshly uniting; and this association is called the conjugal union. Hence it is clear that those men spoke well who said that to consent to marriage is to consent implicitly to fleshly uniting, not explicitly. For the latter ought not to be understood [as given] except in the way that an effect is implicitly contained in its cause: for the power of fleshly uniting, to which one consents, is the cause of the fleshly uniting, as the power of using one’s own possession is the cause of its use.And: “Although the act of fleshly uniting [itself] is not of the essence of marriage, still the power for this is of its essence; for through marriage power over the other’s body is given to both of the spouses with a view to fleshly uniting.”
These things premised, Aquinas maintains it was not inconsistent for Mary and Joseph to know that God wanted them to remain virgins, and yet for them to keep their hearts so open to the divine will that they did not insist on maintaining this virginity “at all costs.”
She [Mary] planned on virginity, unless the Lord should arrange otherwise; hence she entrusted herself to the divine arrangement. To the idea that she consented to fleshly intercourse, one should say that she did not; she consented to marriage directly, but to fleshly intercourse implicitly, so to speak, if God should will it.Again:
While virginity is best in and of itself, nevertheless at that time [before Christ’s birth] marriage was preferred to it because of the expectation of a blessed offspring to come by way of generation; and thus the Blessed Virgin vowed virginity as the thing most excellent and most acceptable to herself, yet not simply speaking, but under a most noble condition—namely this: unless God should ordain otherwise.Hence Mary and Joseph, fully consenting to be each other’s spouses, thereby became spouses, but because it was the Lord’s will that they not unite sexually, they remained “in suspension” as regards the bodily exchange that normally follows upon this consent. In fact, if Mary had not, in wedding Joseph, willed to remain open to the procreation of children by him if God wanted this, then their marriage would have been invalidated through her rejection of one of the essential goods of marriage, the bonum prolis. It may be noted that this curious state of being “in suspension” with regard to legitimate earthly goods typifies the entire eschatological orientation of the Christian condition. For this reason, the marriage of Mary and Joseph is, paradoxically, an exemplar of religious life no less than it is of married life.
In at least one place Aquinas presents the objection that Joseph and Mary’s marriage was imperfect because it lacked children that were an effect of that marriage. His response is that this good was partially present there due to the educatio prolis that took place, and that (to paraphrase) we ought to expect things to be unique in the family of the Son of God. This response helps us to remember that the classic catechisms and codes of canon law always said that the primary purpose of marriage is the “procreation and education of children.” Bringing children into the world is only the first step; the more important step is rearing them and educating them in the Catholic faith, moral virtues, and how to live in one’s society (as well as other subjects of instruction or apprenticeship, depending on circumstances). It is because we implicitly recognize this hierarchy of obligations that we regard adoptive parents as truly parents.
 Summa theologiae III, q. 29, a. 2, citing On Marriage and Concupiscence I; see In IV Sententiarum, d. 30, q. 2, a. 2; Super Matt. 1, lec. 4 [Marietti ed., n. 92]). At In IV Sent. d. 31, q. 1, a. 3 we read: “From the very fact that through the conjugal pact the spouses hand over to each other, in perpetuity, power over one another, it follows that they may not be separated, and so it is that marriage is never found without inseparability; whereas it is found without fidelity and offspring, because the being of a thing does not depend upon its use.”
 In IV Sent. d. 30, q. 2, a. 2, arg. 3.
 In IV Sent. d. 28, q. 1, a. 4.
 In IV Sent. d. 34, q. 1, a. 2, ad 1.
 Super Matt. 1, lec. 4 (Marietti ed., n. 93).
 In IV Sent. d. 30, q. 2, a. 1, qa. 1.
 See In IV Sent. d. 30, q. 2, a. 1, qa. 1, ad 1–3.
 See In IV Sent. d. 30, q. 2, a. 1, qa. 2, ad 2.
 In IV Sent. d. 30, q. 2, a. 2, arg. 4.