Objection 1. It would seem that it can never have been lawful to have several wives. For, according to the Philosopher (Ethic. v, 7), "The natural law has the same power at all times and places." Now plurality of wives is forbidden by the natural law, as stated above (Article 1). Therefore as it is unlawful now, it was unlawful at all times.
Objection 2. Further, if it was ever lawful, this could only be because it was lawful either in itself, or by dispensation. If the former, it would also be lawful now; if the latter, this is impossible, for according to Augustine (Contra Faust. xxvi, 3), "as God is the founder of nature, He does nothing contrary to the principles which He has planted in nature." Since then God has planted in our nature the principle that one man should be united to one wife, it would seem that He has never dispensed man from this.
Objection 3. Further, if a thing be lawful by dispensation, it is only lawful for those who receive the dispensation. Now we do not read in the Law of a general dispensation having been granted to all. Since then in the Old Testament all who wished to do so, without any distinction, took to themselves several wives, nor were reproached on that account, either by the law or by the prophets, it would seem that it was not made lawful by dispensation.
Objection 4. Further, where there is the same reason for dispensation, the same dispensation should be given. Now we cannot assign any other reason for dispensation than the multiplying of the offspring for the worship of God, and this is necessary also now. Therefore this dispensation would be still in force, especially as we read nowhere of its having been recalled.
Objection 5. Further, in granting a dispensation the greater good should not be overlooked for the sake of a lesser good. Now fidelity and the sacrament, which it would seem impossible to safeguard in a marriage where one man is joined to several wives, are greater goods than the multiplication of the offspring. Therefore this dispensation ought not to have been granted with a view to this multiplication.
On the contrary, It is stated (Galatians 3:19) that the Law "was set because of transgressors [Vulgate: 'transgressions']," namely in order to prohibit them. Now the Old Law mentions plurality of wives without any prohibition thereof, as appears from Deuteronomy 21:15, "If a man have two wives," etc. Therefore they were not transgressors through having two wives; and so it was lawful.
Further, this is confirmed by the example of the holy patriarchs, who are stated to have had several wives, and yet were most pleasing to God, for instance Jacob, David, and several others. Therefore at one time it was lawful.
I answer that, As stated above (1, ad 7,8), plurality of wives is said to be against the natural law, not as regards its first precepts, but as regards the secondary precepts, which like conclusions are drawn from its first precepts. Since, however, human acts must needs vary according to the various conditions of persons, times, and other circumstances, the aforesaid conclusions do not proceed from the first precepts of the natural law, so as to be binding in all cases, but only in the majority. for such is the entire matter of Ethics according to the Philosopher (Ethic. i, 3,7). Hence, when they cease to be binding, it is lawful to disregard them. But because it is not easy to determine the above variations, it belongs exclusively to him from whose authority he derives its binding force to permit the non-observance of the law in those cases to which the force of the law ought not to extend, and this permission is called a dispensation. Now the law prescribing the one wife was framed not by man but by God, nor was it ever given by word or in writing, but was imprinted on the heart, like other things belonging in any way to the natural law. Consequently a dispensation in this matter could be granted by God alone through an inward inspiration, vouchsafed originally to the holy patriarchs, and by their example continued to others, at a time when it behooved the aforesaid precept not to be observed, in order to ensure the multiplication of the offspring to be brought up in the worship of God. For the principal end is ever to be borne in mind before the secondary end. Wherefore, since the good of the offspring is the principal end of marriage, it behooved to disregard for a time the impediment that might arise to the secondary ends, when it was necessary for the offspring to be multiplied; because it was for the removal of this impediment that the precept forbidding a plurality of wives was framed, as stated above (Article 1).
Reply to Objection 1. The natural law, considered in itself, has the same force at all times and places; but accidentally on account of some impediment it may vary at certain times and places, as the Philosopher (Ethic. i, 3,7) instances in the case of other natural things. For at all times and places the right hand is better than the left according to nature, but it may happen accidentally that a person is ambidextrous, because our nature is variable; and the same applies to the natural, just as the Philosopher states (Ethic. i, 3,7).
Reply to Objection 2. In a Decretal (De divortiis, cap. Gaudemus) it is asserted that is was never lawful to have several wives without having a dispensation received through Divine inspiration. Nor is the dispensation thus granted a contradiction to the principles which God has implanted in nature, but an exception to them, because those principles are not intended to apply to all cases but to the majority, as stated. Even so it is not contrary to nature when certain occurrences take place in natural things miraculously, by way of exception to more frequent occurrences.
Reply to Objection 3. Dispensation from a law should follow the quality of the law. Wherefore, since the law of nature is imprinted on the heart, it was not necessary for a dispensation from things pertaining to the natural law to be given under the form of a written law but by internal inspiration.
Reply to Objection 4. When Christ came it was the time of the fulness of the grace of Christ, whereby the worship of God was spread abroad among all nations by a spiritual propagation. Hence there is not the same reason for a dispensation as before Christ's coming, when the worship of God was spread and safeguarded by a carnal propagation.
Reply to Objection 5. The offspring, considered as one of the marriage goods, includes the keeping of faith with God, because the reason why it is reckoned a marriage good is because it is awaited with a view to its being brought up in the worship of God. Now the faith to be kept with God is of greater import than the faith to be kept with a wife, which is reckoned a marriage good, and than the signification which pertains to the sacrament, since the signification is subordinate to the knowledge of faith. Hence it is not unfitting if something is taken from the two other goods for the sake of the good of the offspring. Nor are they entirely done away, since there remains faith towards several wives; and the sacrament remains after a fashion, for though it did not signify the union of Christ with the Church as one, nevertheless the plurality of wives signified the distinction of degrees in the Church, which distinction is not only in the Church militant but also in the Church triumphant. Consequently their marriages signified somewhat the union of Christ not only with the Church militant, as some say, but also with the Church triumphant where there are "many mansions" [John 19:2].
Source: Summa Theologica (New Advent)