Shulchan Aruch, Polygamy & Rabbeinu Gershom

See full discussion in Otzer haPoskim volume I from Hebrew Books pages 21 -94

Shulchan Aruch (E.H. 1:9): A man is allowed to marry a number of wives as long is he can afford them. Nevetheless the Sages gave good advice that one should not marry more than four wives in order that each one would have sexual relations at least once each month. In communities where the practice is to have only one wife it is not appropriate to marry an additional wife.

Shulchan Aruch (E.H. 1:10): Rabbeinu Gershom made a cherem against those who married an additional wife. In contrast he didn’t make a ban against those who married their brother’s widow (yevama) who was childless nor an arusa. Rema: If he didn’t want to marry her but to exempt her (Maharik 101). He also didn’t apply it in a case where a mitzva was not being done for example his first wife had no children after 10 years... However others disagree and they assert that the Cherem of Rabbeinu Gershom applies even when it intereferes with doing a mitzvs and even in the case of yebom. Therefore in a case of a married yevam he would be required to do chalitza rather than marrying his brother’s widow (yevama). In a case that she can not be divorced such as she is insane or she refuses to accept the Get from him – then there is reason to be lenient for him to marry someone else. And surely if the first one is only betrothed but doesn’t want to get married or breakup with him. This decree of Rabbeinu Gershom was not universally accepted in all places. Rema:  It doesn’t apply only in those places where it is known not to have been adopted but otherwise it is to assumed that a particular place did accept it. Look at Y.D. 228 concerning a person who travels from a place where it is accepted to one where it was not accepted. Furthermore the Cherem of Rabbeinu Gershom was only decreed until the end of the 5^th milenia. Rema: Nevertheless in all the Ashkenaz lands it was decreed and this practice still applies today and therefore one should not marry two women. And we force with cherem and nidoi anyone who transgressed and married two women – that he should divorce one of them. And some say that today we don’t force someone who has transgressed the Cherem of Rabbeinu Gershom since we already have passed the  5^th milenia but that view is not accepted. There are some who say if his wife becomes an apostate that she is given a get through another party and that is the practice in a number of places. In a place where it is not the practice, there is no need to be strict and it is permitted to marry another woman without divorcing the first.

Shulchan Aruch (E.H. 1:11): It is a good idea to make decrees and bans and shunnings against someone who marries an additional wife.