In this week’s parshah the Torah discusses the halachos of one who has two wives and they both bare children. The Torah says that though one wife is more loved in her husband’s eyes, he may not give her son the firstborn rights if the other wife bore a child first.
Famously, around the year 1000, Rabbeinu Gershom issued a series of takanos, including a prohibition on polygamy.
The Acharonim were bothered by the following question: the Taz (in Orach Chaim 588 and Yorah Deah 117) says that the Rabanan may not forbid something that the Torah explicitly permitted. In truth this concept is found in Tosafos in Baba Metzia 70b. Therefore, they ask, how can Rabbeinu Gershom forbid marrying more than one wife, when the Torah explicitly permits it in this parshah?
Similarly they ask on another takanah of Rabbeinu Gershom, not to divorce a woman against her will. The Torah explicitly says that a husband may divorce his wife if he so pleases. How can Rabbeinu Gershom forbid these things when the Torah seemingly explicitly permits them?
One could suggest that Rabbeinu Gershom fundamentally disagrees with this concept and opines that the Rabanan may forbid something even if the Torah explicitly permits it. However, since we do not find any Rishonim who openly disagree with this concept, it is difficult to assume that Rabbeinu Gershom did.
I believe that the simplest answer to this question is that Rabbeinu Gershom did not enact a prohibition against marrying more than one wife or divorcing her against her will. Rather, he made a cherem against one who does these things. This means that one who acts in this manner should be put in cherem. However, it is not forbidden to act this way. Tosafos and the Taz only said that the Rabanan may not create an issur to do something which the Torah explicitly permitted. They never said that the Rabanan may not issue a cherem against such an act. This answer is brought in the Chasam Sofer (likutim chelek 6, siman 56).
To this extent, the Beis Shmuel (Even Ha’ezer 1:21) says that if a doubt arises concerning the cherem of Rabbeinu Gershom one may be lenient. This is because Rabbeinu Gershom never intended to create an issur; rather he wanted to prevent fighting.
Rav Shlomo Kluger (Teshuvos Tuv Tam Va’da’as 1:22) says that the Torah never explicitly permitted one to marry more than one wife. The Torah only discussed the halachah of when someone was in this situation. The Taz was discussing making a bris milah on Shabbos, where the Torah explicitly commands that we desecrate the Shabbos in order to make the bris milah. There the Rabanan do not have the authority to enact a decree forbidding such an action. Similarly, Tosafos in Baba Metzia was discussing lending to goyim with interest, something that the Torah explicitly permits. Tosafos says that the Rabanan would not be able to enact a decree forbidding such an action. However, here the Torah never explicitly said that one should or could marry more than one wife.
Rav Kluger continues to say that in our case there is one opinion in the Gemara in Yevamos (23a) that explains that the “hated” wife refers to a woman who it was forbidden to marry. She is halachicly “hated” and the pasuk is coming to teach us that her son will nonetheless receive the firstborn rights. We see that according to this opinion there is no source that the Torah ever permitted one to marry more than one woman.
The Minchas Elazar (1:62) suggests another solution to the original question. He first asks why Rabbeinu Gershom stipulated that this cherem last only until the end of the fifth millennium (see Shulchan Aruch, Even Ha’ezer, siman 1:10. The Rama says that communities have accepted this upon themselves even afterwards). The Minchas Elazar says that Rabbeinu Gershom did not enact his cherem for all generations because he couldn’t, since the Torah had permitted marrying two wives. He explains that the Rabanan cannot make a new prohibition that will be in affect forever when the Torah permitted something. They can, however, make a temporary prohibition. Therefore Rabbeinu Gerhsom only made his cherem for a short period of time. Afterwards the communities have accepted upon themselves to continue this on their own accord.