Objection 1. It would seem that if an oath be added to a consent that is expressed in words of the future tense it makes a marriage. For no one can bind himself to act against the Divine Law. But the fulfilling of an oath is of Divine law according to Matthew 5:33, "Thou shalt perform thy oaths to the Lord." Consequently no subsequent obligation can relieve a man of the obligation to keep an oath previously taken. If, therefore, after consenting to marry a woman by words expressive of the future and confirming that consent with an oath, a man binds himself to another woman by words expressive of the present, it would seem that none the less he is bound to keep his former oath. But this would not be the case unless that oath made the marriage complete. Therefore an oath affixed to a consent expressed in words of the future tense makes a marriage.
Objection 2. Further, Divine truth is stronger than human truth. Now an oath confirms a thing with the Divine truth. Since then words expressive of consent in the present in which there is mere human truth complete a marriage, it would seem that much more is this the case with words of the future confirmed by an oath.
Objection 3. Further, according to the Apostle (Hebrews 6:16), "An oath for confirmation is the end of all... controversy"; wherefore in a court of justice at any rate one must stand by an oath rather than by a mere affirmation. Therefore if a man consent to marry a woman by a simple affirmation expressed in words of the present, after having consented to marry another in words of the future confirmed by oath, it would seem that in the judgment of the Church he should be compelled to take the first and not the second as his wife.
Objection 4. Further, the simple uttering of words relating to the future makes a betrothal. But the addition of an oath must have some effect. Therefore it makes something more than a betrothal. Now beyond a betrothal there is nothing but marriage. Therefore it makes a marriage.
On the contrary, What is future is not yet. Now the addition of an oath does not make words of the future tense signify anything else than consent to something future. Therefore it is not a marriage yet.
Further, after a marriage is complete, no further consent is required for the marriage. But after the oath there is yet another consent which makes the marriage, else it would be useless to swear to a future marriage. Therefore it does not make a marriage.
I answer that, An oath is employed in confirmation of one's words; wherefore it confirms that only which is signified by the words, nor does it change their signification. Consequently, since it belongs to words of the future tense, by their very signification, not to make a marriage, since what is promised in the future is not done yet, even though an oath be added to the promise, the marriage is not made yet, as the Master says in the text (Sent. iv, D, 28).
Reply to Objection 1. The fulfilling of a lawful oath is of Divine law, but not the fulfilling of an unlawful oath. Wherefore if a subsequent obligation makes that oath unlawful, whereas it was lawful before, he who does not keep the oath he took previously does not disobey the Divine law. And so it is in the case in point; since he swears unlawfully who promises unlawfully; and a promise about another's property is unlawful. Consequently the subsequent consent by words of the present, whereby a man transfers the power over his body to another woman, makes the previous oath unlawful which was lawful before.
Reply to Objection 2. The Divine truth is most efficacious in confirming that to which it is applied. Hence the Reply to the Third Objection is clear.
Reply to Objection 4. The oath has some effect, not by causing a new obligation, but confirming that which is already made, and thus he who violates it sins more grievously.
Source: Summa Theologica (New Advent)