Objection 1. It seems that it could not be lawful by dispensation to put away a wife. For in marriage anything that is opposed to the good of the offspring is against the first precepts of the natural law, which admit of no dispensation. Now such is the putting away of a wife, as stated above (Article 1). Therefore, etc.
Objection 2. Further, a concubine differs from a wife especially in the fact that she is not inseparably united. But by no dispensation could a man have a concubine. Therefore by no dispensation could he put his wife away.
Objection 3. Further, men are as fit to receive a dispensation now as of old. But now a man cannot receive a dispensation to divorce his wife. Neither, therefore, could he in olden times.
On the contrary, Abraham carnally knew Agar with the disposition of a husband towards his wife, as stated above (65, 5, ad 2,3). Now by Divine command he sent her away, and yet sinned not. Therefore it could be lawful by dispensation for a man to put away his wife.
I answer that, In the commandments, especially those which in some way are of natural law, a dispensation is like a change in the natural course of things: and this course is subject to a twofold change. First, by some natural cause whereby another natural cause is hindered from following its course: it is thus in all things that happen by chance less frequently in nature. In this way, however, there is no variation in the course of those natural things which happen always, but only in the course of those which happen frequently. Secondly, by a cause altogether supernatural, as in the case of miracles: and in this way there can be a variation in the course of nature, not only in the course which is appointed for the majority of cases, but also in the course which is appointed for all cases, as instanced by the sun standing still at the time of Josue, and by its turning back at the time of Ezechias, and by the miraculous eclipse at the time of Christ's Passion [Joshua 10:14; 2 Kings 20:10; Isaiah 38:8; Matthew 27:15]. In like manner the reason for a dispensation from a precept of the law of nature is sometimes found in the lower causes, and in this way a dispensation may bear upon the secondary precepts of the natural law, but not on the first precepts because these are always existent as it were, as stated above (Question 65, Article 1) in reference to the plurality of wives and so forth. But sometimes this reason is found in the higher causes, and then a dispensation may be given by God even from the first precepts of the natural law, for the sake of signifying or showing some Divine mystery, as instanced in the dispensation vouchsafed to Abraham in the slaying of his innocent son. Such dispensations, however, are not granted to all generally, but to certain individual persons, as also happens in regard to miracles. Accordingly, if the indissolubility of marriage is contained among the first precepts of the natural law, it could only be a matter of dispensation in this second way; but, if it be one of the second precepts of the natural law, it could be a matter of dispensation even in the first way. Now it would seem to belong rather to the secondary precepts of the natural law. For the indissolubility of marriage is not directed to the good of the offspring, which is the principal end of marriage, except in so far as parents have to provide for their children for their whole life, by due preparation of those things that are necessary in life. Now this preparation does not pertain to the first intention of nature, in respect of which all things are common. And therefore it would seem that to put away one's wife is not contrary to the first intention of nature, and consequently that it is contrary not to the first but to the second precepts of the natural law. Therefore, seemingly, it can be a matter of dispensation even in the first way.
Reply to Objection 1. The good of the offspring, in so far as it belongs to the first intention of nature, includes procreation, nourishment, and instruction, until the offspring comes to perfect age. But that provision be made for the children by bequeathing to them the inheritance or other goods belongs seemingly to the second intention of the natural law.
Reply to Objection 2. To have a concubine is contrary to the good of the offspring, in respect of nature's first intention in that good, namely the rearing and instruction of the child, for which purpose it is necessary that the parents remain together permanently; which is not the case with a concubine, since she is taken for a time. Hence the comparison fails. But in respect of nature's second intention, even the having of a concubine may be a matter of dispensation as evidenced by Hosea 1.
Reply to Objection 3. Although indissolubility belongs to the second intention of marriage as fulfilling an office of nature, it belongs to its first intention as a sacrament of the Church. Hence, from the moment it was made a sacrament of the Church, as long as it remains such it cannot be a matter of dispensation, except perhaps by the second kind of dispensation.
Source: Summa Theologica (New Advent)