I stole this from another poster in the polygamy thread for reference:
St. Thomas Aquinas, Summa Contra Gentiles
“I answer that, As stated above (A[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 (A[1]).”