Objection 1. It would seem that consent given in words expressive of the future makes a marriage. For as present is to present, so is future to future. But consent given in words expressive of the present makes a marriage in the present. Therefore consent given in words expressive of the future makes a marriage in the future.
Objection 2. Further, in other civil contracts, just as in matrimony, a certain obligation results from the words expressing consent. Now in other contracts it matters not whether the obligation is effected by words of the present or of the future tense. Therefore neither does it make any difference in matrimony.
Objection 3. Further, by the religious vow man contracts a spiritual marriage with God. Now the religious vow is expressed in words of the future tense, and is binding. Therefore carnal marriage also can be effected by words of the future tense.
On the contrary, A man who consents in words of the future tense to take a particular woman as his wife, and after, by words of the present tense, consents to take another, according to law must take the second for his wife (cap. Sicut ex Litteris, De spons. et matr.). But this would not be the case if consent given in words of the future tense made a marriage, since from the very fact that his marriage with the one is valid, he cannot, as long as she lives, marry another. Therefore consent given in words of the future tense does not make a marriage.
Further, he who promises to do a certain thing does it not yet. Now he who consents in words of the future tense, promises to marry a certain woman. Therefore he does not marry her yet.
I answer that, The sacramental causes produce their effect by signifying it; hence they effect what they signify. Since therefore when a man expresses his consent by words of the future tense, he does not signify that he is marrying, but promises that he will marry, it follows that a consent expressed in this manner does not make a marriage, but a promise [sponsionem] of marriage, and this promise is known as a betrothal [sponsalia].
Reply to Objection 1. When consent is expressed in words of the present tense, not only are the words actually present, but consent is directed to the present, so that they coincide in point of time; but when consent is given in words of the future tense, although the words are actually present, the consent is directed to a future time, and hence they do not coincide in point of time. For this reason the comparison fails.
Reply to Objection 2. Even in other contracts, a man who uses words referring to the future, does not transfer the power over his property to another person--for instance if he were to say "I will give thee"--but only when he uses words indicative of the present.
Reply to Objection 3. In the vow of religious profession it is not the spiritual marriage itself that is expressed in words which refer to the future, but an act of the spiritual marriage, namely obedience or observance of the rule. If, however, a man vow spiritual marriage in the future, it is not a spiritual marriage, for a man does not become a monk by taking such a vow, but promises to become one.
Source: Summa Theologica (New Advent)