The End of Jewish (Ashkenaz) Polygamy

(9) A man may marry a number of women provided that he has the means to sustain them, and what we find is that the sages gave worthy suggestion that a man marry no more than four women so that he may fulfill his sexual obligation once a month. In a place where they are accustomed to only marry one woman they are not permitted to marry another woman. Note: it is forbidden to marry two women in two places.

(10) Rabbenu Gershom [ben Yehuda] decreed excommunication on one who marries [another woman] in addition to his wife, though he did not decree excommunication in [the case of] a yebama [a widowed sister-in-law to whom one is obligated in levirate marriage], as well as in [the case of] an erusa [a betrothed woman]. Rem"a: If he only wishes to marry in order to divorce (Maharik [Moreinu HaRav Yosef Colon, Trabotto] chapter 101). And this is the rule wherever there is a mitzvah that supersedes, for example, when one cohabits with his wife for ten years and she has not borne a child ([The] Mordechai [ben Hillel] [on Talmud Bavli Yevamot] chapter HaCholetz [ie: 4th chapter], Rashba [Rav Shlomo Ben Avraham Aderet chapter 280 and Maharam Padua [Meir ben Yitzchak Katzenellenbogen] chapter 19). It is true that there are those who disagree and think that Rabenu Gershom's [decree of] excommunication applies even where there is a mitzvah [to marry] and even with regard to yibum [levirate marriage], and one would need to perform chalitzah [the releasing of those obligated in levirate marriage] (Mordechai's commentary on Yevamot, and Mordechai's commentary on Ketuvot, and so wrote Nimukei Yosef [Yosef ibn Habib] [in] chapter HaCholetz). And where the first [wife] is not divorced, for example, that she has become demented or that he may lawfully divorce her but she does not wish to accept a get [bill of divorce] from him, there is [room] to be lenient, to allow him to marry another (so is indicated in the responsum of the Rasbha]. And even more so if she is betrothed but does not want to marry him or to divorce him. And his decree has not spread to all countries. Rem"a: And [this refers] specifically to a place where it is known his decree has not spread, but [otherwise], by default, [the decree] is applicable in all places (Responsum of RI Minz chapter 10). And see Yoreh De'ah chapter 228 [with regard to] if one goes from a place where they are customarily stringent to a place where they are customarily lenient. And he only decreed excommunication up until the end of the fifth millennium. Rem"a: And in any case, in all those countries the decree and the custom stands, and one does not marry two wives, and we force, with excommunications and expulsions, one who has transgressed and married two wives to divorce one of them. And some say that in our day we should not force one who has transgressed Rabbenu Gershom's decree of excommunication [to divorce] (Beit Yosef in chapter 76), because the fifth millennium has already come to an end (ibid., in the name of Maharik chapter 110). And our custom is not so. One whose wife apostatized, he should provide her with a get through another's hand, and marry another [wife], and this is the custom is several places (Piskei Mahari [Israel Isserlin] chapter 256), and in a place where there is no custom [regarding an apostatizing wife], there is no [need] to be stringent, and it is permitted to marry another without divorcing the first (ibid., customs of Rinus [Rhine, vs. Austria]).