Thomas Aquinas

“ Consequently it must be said that for a mortal sin it is sufficient to give consent to something present, whereas in a betrothal the consent is to something future; and greater discretion of reason is required for looking to the future than for consenting to one present act. Wherefore a man can sin mortally before he can bind himself to a future obligation.
Reply to Objection 3: Regarding the age for the marriage contract a disposition is required not only on the part of the use of reason, but also on the part of the body, in that it is necessary to be of an age adapted to procreation. ”

“ However, properly speaking, espousals are so called from the promise [sponsione] in the first sense, because espousals are a kind of sacramental annexed to matrimony, as exorcism to baptism.
Art. 2 - Whether seven years is fittingly assigned as the age for betrothal? Objection 1: It would seem that seven years is not fittingly assigned as the age for betrothal. For a contract that can be formed by others does not require discretion in those whom it concerns. Now a betrothal can be arranged by the parents without the knowledge of either of the persons betrothed. ”

“ Further, if persons become betrothed before they are seven years old, and subsequently after the age of seven and before the age of maturity renew their promise in words expressive of the present, they are reckoned to be betrothed. Now this is not by virtue of the second contract, since they intend to contract not betrothal but marriage. Therefore it is by the virtue of the first; and thus espousals can be contracted before the age of seven.
Objection 6: Further, when a thing is done by many persons in common, if one fails he is supplied by another, as in the case of those who row a boat. ”

“ For if I have promised a thing to someone I cannot lawfully pledge it to someone else. Now he who betroths himself promises his body to the woman. Therefore he cannot make a further offering of himself to God in religion. ”

“ If the parties are betrothed by another person before they reach the age of puberty, either of them or both can demur; wherefore in that case the betrothal does not take effect, so that neither does any affinity result therefrom. Hence a betrothal made between certain persons by some other takes effect, in so far as those between whom the betrothal is arranged do not demur when they reach the proper age, whence they are understood to consent to what others have done. ”

“ Further, whoever promises a thing must be compelled to fulfill his promise. But those who have contracted a betrothal are not compelled by the Church to fulfill the marriage. Therefore a betrothal is not a promise. ”

“ Sometimes, however, the condition appended is the payment of a sum of money by way of penalty, and then, since marriage should be free, such a condition does not hold, nor can such a penalty be exacted from a person who is unwilling to fulfill the promise of marriage. ”