At what point did polygamy become considered sinful in Jewish and Christian history, and why?
Ah, one of my pet issues, ever since becoming hooked a few years ago on Big Love (now ended).
Judaism permitted polygamy until 960 CE/AD, when Rabbenu Gershom issued a ban on it (for Ashkenazi Jews only, since European Jews were the only ones he had jurisdiction over). The ban was intended to last only 500 years, but by then most Ashkenazi Jews had become used to it.
Interestingly, it was not done for religious reasons; it was done to prevent antisemitism, because it was felt that if Jews could continue having plural wives when Catholics could not, Jew hatred would arise.
In the 1950s, when the zionist state initiated Operation Magic Carpet to airlift Jews from Morocco, the Ashkenazi Jews in the Holy Land were aghast to see Moroccan Jewish men descending the plane steps with their often 4 wives each in tow, all fussing over them! What a shanda! lol
A law was passed that banned future polygamy, though any Sephardic Jews with multiple wives at the time were allowed to keep them.