Objection 1. It would seem that a second marriage is not a sacrament. For he who repeats a sacrament injures the sacrament. But no sacrament should be done an injury. Therefore if a second marriage were a sacrament, marriage ought nowise to be repeated.
Objection 2. Further, in every sacrament some kind of blessing is given. But no blessing is given in a second marriage, as stated in the text (Sent. iv, D, 42). Therefore no sacrament is conferred therein.
Objection 3. Further, signification is essential to a sacrament. But the signification of marriage is not preserved in a second marriage, because there is not a union of only one woman with only one man, as in the case of Christ and the Church. Therefore it is not a sacrament.
Objection 4. Further, one sacrament is not an impediment to receiving another. But a second marriage is an impediment to receiving orders. Therefore it is not a sacrament.
On the contrary, Marital intercourse is excused from sin in a second marriage even as in a first marriage. Now marital intercourse is excused [Cf. 69, 1 by the marriage goods which are fidelity, offspring, and sacrament. Therefore a second marriage is a sacrament.
Further, irregularity is not contracted through a second and non-sacramental union, such as fornication. Yet irregularity is contracted through a second marriage. Therefore it is a sacramental union.
I answer that, Wherever we find the essentials of a sacrament, there is a true sacrament. Wherefore, since in a second marriage we find all the essentials of the sacrament of marriage (namely the due matter--which results from the parties having the conditions prescribed by law--and the due form, which is the expression of the inward consent by words of the present), it is clear that a second marriage is a sacrament even as a first.
Reply to Objection 1. This is true of a sacrament which causes an everlasting effect: for then, if the sacrament be repeated, it is implied that the first was not effective, and thus an injury is done to the first, as is clear in all those sacraments which imprint a character. But those sacraments which have not an everlasting effect can be repeated without injury to the sacrament, as in the case of Penance. And, since the marriage tie ceases with death, no injury is done to the sacrament if a woman marry again after her husband's death.
Reply to Objection 2. Although the second marriage, considered in itself, is a perfect sacrament, yet if we consider it in relation to the first marriage, it is somewhat a defective sacrament, because it has not its full signification, since there is not a union of only one woman with only one man as in the marriage of Christ with the Church. And on account of this defect the blessing is omitted in a second marriage. This, however, refers to the case when it is a second marriage on the part of both man and woman, or on the part of the woman only. For if a virgin marry a man who has had another wife, the marriage is blessed nevertheless. Because the signification is preserved to a certain extent even in relation to the former marriage, since though Christ has but one Church for His spouse, there are many persons espoused to Him in the one Church. But the soul cannot be espoused to another besides Christ, else it commits fornication with the devil. Nor is there a spiritual marriage. For this reason when a woman marries a second time the marriage is not blessed on account of the defect in the sacrament.
Reply to Objection 3. The perfect signification is found in a second marriage considered in itself, not however if it be considered in relation to the previous marriage, and it is thus that it is a defective sacrament.
Reply to Objection 4. A second marriage in so far as there is a defect in the sacrament, but not as a sacrament, is an impediment to the sacrament of Order.
Source: Summa Theologica (New Advent)