It is a difficult question, but I am going to try to explain Aquinas’ position as I understand it. I apologize for the long post.
St. Thomas describes it this way (Summa Theologica, Supplement, Q65). The first principle of marriage is the generation (birth) and education of children. It is the first principle because it is the most generic: reproduction via sexuality is something that we share in common with animals. Now, there are secondary principles also, and these are certain goods that are more particular to man, for example, the care of the spouses for each other. Even more specifically, there is an aspect of marriage for Christian believers: that marriage is an image of the marriage of Christ and the Church.
Remember that the first principle of a thing must also be born in mind, even when secondary principles are being treated.
Polygamy, St. Thomas says, is contrary to human nature…but it does not, in fact, hinder the first principle of marriage, which as we said is the birth and education of children. However, polygamy does hinder the secondary aspects: for example, the good of the spouses is harmed when two women are living with one man ("…there cannot easily be peace in a family where several wives are joined to one husband…"). And, in the case of the Christian dimension (marriage as the reflection of Christ and the Church), polygamy completey destroys this good (there is one only Christ and one Church).
The final important piece of information is that there was, in the Old Testament, a real and critical need that pertained directly (and only) to the first principle of marriage: before Christ and the fullness of the grace of God being manifested in the world, “the worship of God was spread and safeguarded by a carnal propagation” (a2). In other words, the continuation of salvation history, in the Old Testament, dependend literally on the people of God having children and so continuing to exist.
The conclusion is this. First, there was no explicit condemnation of polygamy in divine law from the Old Testament (the commandments given to the Jews). Second, there was a real and critical need that pertained directly and only to the first principle of marriage - namely, that the people of God needed to be increased in order to spread the worship of God. Third, because polygamy doesn’t violate the first principle of marriage - the principle which the critical problem of the propagation of the Jewish people directly applied to - God deemed it acceptable to grant an “exception” to the Old Testament patriarchs who practiced polygamy so that it was lawful for them to do so. The key point is: this “exception” was granted because it was necessary for the propagation of the Jewish people in order for the worship of God to continue in history.
That such “exceptions” no longer exist is evident on all counts: first, because of the words of Jesus, there is now divine law prohibiting polygamy; second, because Jesus has come, the grace of God has fully entered into the world, so the propagation of the faith is no longer dependent on carnal generation.