The sacrament of marriage can only occur between one man and one woman. And natural marriage, from the beginning, was intended by God, in its fullness of perfection, to occur only between one man and one woman (Mt 19).
But, in the Old Testament, the Patriarchs sometimes had more than one wife. And the Old Testament law speaks of a man having more than one wife, without condemnation. So the question arises, was it ever morally licit under natural law, for unbaptized persons and merely natural marriage, for a man to have more than one wife?
In the Summa Theologica, Saint Thomas Aquinas answers that it was morally licit.
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.
He explains two reasons for this licitness. First, the primary end of marriage, under natural law, is the generation and education of offspring. If a man has multiple wives, they can more easily and more quickly be fruitful and multiply, “in order to ensure the multiplication of the offspring to be brought up in the worship of God” (Summa). The secondary end of marriage, he says, is the “community of works”, whereby husband and wife have a loving lifelong union in which they cooperate together. However, a plurality of wives is only in some ways contrary to that purpose of marriage, and in other ways compatible with it.
If a plurality of wives were contrary to the primary end of marriage, Thomas says, then it would be always immoral (i.e. intrinsically evil). But since it is only contrary to the secondary end, and then only to some extent, it is permissible in some circumstances (not intrinsically evil).
The second reason that Thomas gives for the licitness of polygamy, specifically in the case of the Old Testament, is a dispensation from God so that the children of Abraham could be like the sand of the sea. A man with multiple wives, as was the case with Jacob, could produce holy offspring to increase the number of the chosen people more quickly.
This second reason justifies the polygamy (actually, polygyny) of cases in the Old Testament. However, the first reason justifies a plurality of wives (but not of husbands) in any natural law cases, that is, when the married persons are not baptized.
However, as is clear from Jesus’ explanation of Genesis, the plan of God from the beginning was for marriage to be one man and one woman. Thus, even the Old Testament cases of polygamy, while not intrinsically evil, are contrary to the fullness of marriage as intended by God.
And that is why the Sacrament of Marriage never permits polygamy. Natural marriage is raised to the fullness of its perfection by becoming a Sacrament, and so the Sacrament of holy Matrimony is also a natural marriage.
Please take a look at this list of my books and booklets, and see if any topic interests you.