Objection 1. It would seem that consent given secretly in words of the present does not make a marriage. For a thing that is in one person's power is not transferred to the power of another without the consent of the person in whose power it was. Now the maid is in her father's power. Therefore she cannot by marriage be transferred to a husband's power without her father's consent. Wherefore if consent be given secretly, even though it should be expressed in words of the present, there will be no marriage.
Objection 2. Further, in penance, just as in matrimony, our act is as it were essential to the sacrament. But the sacrament of penance is not made complete except by means of the ministers of the Church, who are the dispensers of the sacraments. Therefore neither can marriage be perfected without the priest's blessing.
Objection 3. Further, the Church does not forbid baptism to be given secretly, since one may baptize either privately or publicly. But the Church does forbid the celebration of clandestine marriages (cap. Cum inhibitio, De clandest. despons.). Therefore they cannot be done secretly.
Objection 4. Further, marriage cannot be contracted by those who are related in the second degree, because the Church has forbidden it. But the Church has also forbidden clandestine marriages. Therefore they cannot be valid marriages.
On the contrary, Given the cause the effect follows. Now the sufficient cause of matrimony is consent expressed in words of the present. Therefore whether this be done in public or in private the result is a marriage.
Further, wherever there is the due matter and the due form of a sacrament there is the sacrament. Now in a secret marriage there is the due matter, since there are persons who are able lawfully to contract--and the due form, since there are the words of the present expressive of consent. Therefore there is a true marriage.
I answer that, Just as in the other sacraments certain things are essential to the sacrament, and if they are omitted there is no sacrament, while certain things belong to the solemnization of the sacrament, and if these be omitted the sacrament is nevertheless validly performed, although it is a sin to omit them; so, too, consent expressed in words of the present between persons lawfully qualified to contract makes a marriage, because these two conditions are essential to the sacrament; while all else belongs to the solemnization of the sacrament, as being done in order that the marriage may be more fittingly performed. Hence if these be omitted it is a true marriage, although the contracting parties sin, unless they have a lawful motive for being excused. [Clandestine marriages have since been declared invalid by the Council of Trent (sess. xxiv). It must be borne in mind that throughout the treatise on marriage St. Thomas gives the Canon Law of his time.]
Reply to Objection 1. The maid is in her father's power, not as a female slave without power over her own body, but as a daughter, for the purpose of education. Hence, in so far as she is free, she can give herself into another's power without her father's consent, even as a son or daughter, since they are free, may enter religion without their parent's consent.
Reply to Objection 2. In penance our act, although essential to the sacrament, does not suffice for producing the proximate effect of the sacrament, namely forgiveness of sins, and consequently it is necessary that the act of the priest intervene in order that the sacrament be perfected. But in matrimony our acts are the sufficient cause for the production of the proximate effect, which is the marriage bond, because whoever has the right to dispose of himself can bind himself to another. Consequently the priest's blessing is not required for matrimony as being essential to the sacrament.
Reply to Objection 3. It is also forbidden to receive baptism otherwise than from a priest, except in a case of necessity. But matrimony is not a necessary sacrament: and consequently the comparison fails. However, clandestine marriages are forbidden on account of the evil results to which they are liable, since it often happens that one of the parties is guilty of fraud in such marriages; frequently, too, they have recourse to other nuptials when they repent of having married in haste; and many other evils result therefrom, besides which there is something disgraceful about them.
Reply to Objection 4. Clandestine marriages are not forbidden as though they were contrary to the essentials of marriage, in the same way as the marriages of unlawful persons, who are undue matter for this sacrament; and hence there is no comparison.
Source: Summa Theologica (New Advent)