This is an interesting point and well worth exploring here.
Here are some quick references for those, like Blue Horizon, who may need to refresh their memory or who, like me, had no idea what Thomas said on the subject of polygamy:
From the first, here are Thomas’ three tests:
bringing forth and the education of children.
the common life enjoyed by the spouses.
sacramental sign given by the fidelity of one man to one woman.
Polygamy might seem to satisfy the first test if, as is usually the case, the husband is wealthy and can afford to raise all the children. However, this presumes that the education can be subcontracted out. To some extent that’s obviously true. But there is another sense in which it is not: it is well known that in modern society absent fathers are a major cause of social pathology. If, as commonly practiced, each wife may have her own home (or her own floor in a common home) forming, essentially, a distinct household, you have almost the equivalent of single mothers save for what time the father spends with that wife and family. If, on the other hand, the wives and children are combined then you have confusion over authority of the mothers. Multiple wives betting multiple children is not equivalent to one wife having very many children.
The same holds true for the second test. How close are a husband and wife likely to be if the wife is one of many? How common is their life, really? This is especially the case when, again as commonly practiced, the ages of the wives vary with new wives coming in as the other wives grow older with, often, the oldest wives becoming neglected.
Thomas notes that polygamy fails the third test but this is the least persuasive to me. Jesus did declare that marriage is one man and one woman but that begs the question why? It would seem arbitrary if polygamy fulfilled all other tests. Thomas notes that God had nothing ill to say about the polygamy of the patriarchs.
But I claim that Jesus opposed polygamy just as he opposed divorce, which also was permitted up until then and that he was, in this instance, teaching a tighter, more natural marriage. Of coures, if polygamy were truly unnatural then this ought to have been discovered without revelation and, indeed, polygamy remains exceptional outside of Christianity even where it is officially sanctioned because it has some serious problems.