Thesis: The Torah, views monogamy as an ideal, but permitted polygamy as a concession to reality, mainly economic reality. The ancient Rabbis shared this view.
To prove this, I will need to demonstrate positive visions of monogamy in the Torah and Chazal, negative views of polygamy, and evidence for why the Torah permitted it nonetheless. It will also need to explain why later Rabbis could forbid polygamy and not have to worry about the economic consequences.
Torah – pro-monogamy:
1) The creation of man and woman as two individuals who were originally one, and that the Torah presumes only one wife per man when describing marriage in the creation story, implies that a monogamous relationship is the ideal.
2) Shir Ha-shirim, the love story understood as a metaphor for God’s relationship with Israel, but using imagery of an erotic relationship between a man and a woman, is monogamous.
3) In the prophers, especially Ezekiel, the theme of marriage and divorce between God and Israel constantly recurs. There is no indication, ever, that God, who is considered the ‘man’ in the metaphors, has more than one ‘Chosen People’. In fact, it seems absurd to suggest otherwise. That marriage can be a metaphor for chosenness implies that marriage is ideal constructed as a monogamous relationship.
1) The positive example of polygamy in the Torah: none.
2) Neutral examples: Lamech, Nachor, Esav (I’m sure there are more, but like these two, they’re peripheral characters at best).
3) Negative examples: Avraham, Yaakov, David, Solomon, Elkanah. Major players all.
4) When the Torah describes the laws of birthright for a man with two wives, it begins by saying, “When a man has two wives, one beloved and one hated…”
5) The Biblical and Rabbinic word for co-wives is ‘Tzatah”, which literally means ‘enemy’.
In Rabbinic works, there are all kinds of statements about marriage, wives, etc. There are too many to begin listing. I don’t know of any view that’s pro-polygamy in Chazal.
The economic benefits of polygamy:
1) In a tribal society which fights wars, there will be more women than men.
2) Women did not have the earning power to sustain themselves. Thus, they were always supported either by their fathers or their husbands. This responsibility is a central portion of the ketubah – marriage contract. It stipulates that the husband must undertake to feed, clothe, and shelter his wife. In return, anything which she produces goes to him. The Gemara in Ketubot explicitly allows her to opt out of this condition if she wishes. After all, the condition was instituted for her benefit. If she’s independently wealthy or has sufficient earning power, she need not take his support in exchange for whatever she profits. In other words, the default reality was that a woman would not be earning enough to support herself, so marriage was an economically advantageous option for her. If each man would only be permitted to marry one woman, there would be a preponderance of poor unmarried women.
3) There’s a Gemara, and I can’t for the life of me remember where it is. It describes how during a year of famine, a Rabbi who was a Kohein and received the Terumah of many people, married five widows so that they wouldn’t starve. He had an abundance of Terumah’, which is only permissible for a Kohein and his wife (wives), slaves, and children to eat. In this instance, extreme but, I believe, paradigmatic, polygamy is clearly a concession to economic realities.
[UPDATE: it's a Tosefta, Ketubot 5:1, about R' Tarfon, who betrothed 300 (!) women so that they could eat Terumah. Thanks, Bob, for the reference].
I can’t nail this one down. If my hypothesis is correct, though, then the conditions in Ashkenaz in early medieval times would have afforded women more financial independence. Why? Was the greater social security? Were inheritance laws different? Was the fact that the economy was more mercantile in the areas of Jewish settlement, so there were more economic opportunities open to women? There are many possible explanations. It should also be noted that, in general, women enjoyed a higher social status in Ashkenaz than in Sephardic lands at this time (ex., more examples of learned women, reports of women making a mezuman or laying tefillin, the issue of ‘Nashim Chashuvos’ that Tosafos discuss in Pesachim, other decrees that improve the lot of women, such as consent to divorce, etc.
If anyone has specific ideas on what could have been the immediate catalyst for this legislation, I’m all ears.
What I've tried to demonstrate here is that there is a moral progression through history which goes beyond the law as proscribed by the Torah, but does so by developing values which are latent within the Torah. I believe this to be the true essence of chumrah, as I posted a while back.
Thus, it's possible to be 'frummer' than the law of the Torah, but only by further developing the spirit of the law.
This phenomenon can have much broader application - think of slavery, vegetarianism, and the list can go on.
It's a relatively narrow and discrete example of 'progressive revelation', but gives a good model of how it can work.