C. The reasons behind the ban:[1]

In the original wording of the ban that was placed by Rabbeinu Gershom no reason is mentioned as for why the ban was placed. Accordingly, the reason behind the ban has been a source of speculation amongst the Rishonim and Poskim with several reasons being offered, and with some admitting that they indeed do not know the reason behind the institution.[2]

1. Financial burden: Some Poskim[3] suggest that the reason behind the ban is due to the financial burden that having many wives and children would entail for the husband, and in times of exile when the economic state of Jews is precarious, there is real worry that he would not be able to support them all.
2. Prevent Domestic abuse: Other Poskim[4] suggest that the ban was placed for the benefit of the first wife to prevent her from being abused by aristocratic husbands.
3. Prevent infighting: Other Poskim[5] suggest that in addition, it is possible that the ban was placed also for the benefit of the husband, to prevent strife and discord from breaking out in his home. Other Poskim[6] suggest the ban was placed in order to prevent the co-wives from fighting and hating each other.
4. Prevent Antisemitism: Other Poskim[7] rule that the main reason for the ban was to protect one from potential danger, as Christian Europe viewed the marriage to two wives as adulterous and one who would do so would put his life in jeopardy.
5. Prevent Incest: Other Poskim[8] suggest the ban was placed to prevent siblings from accidentally marrying each other.
6. Time of Churban: Other Poskim[9] suggested ban was placed in light of the fact we are now in exile and it is not proper for one to be married to more than one wife, as even the marriage to one wife was challenged in the Talmud^[10] during times of exile.

No Issur: Most Poskim[11] rule that the ban was not placed due to worry of transgression [i.e. Derara Dissur]. Other Poskim[12], however, rule that the ban was placed due to worry of prohibition. The practical ramification is regarding being lenient in a case of doubt, as explained next in D.[13]

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[1] See Encyclopedia Talmudit Vol. 17 p. 379

[2] See Rashba 3:446; Shut Haran 48

[3] Maharam Padvah 14; Maharim Paduva 16; Mishkanos Yaakov 1

[4] Shut Haran 48; Darkei Moshe 1:9 and Maharik Shoresh 101 in name of Rashba “due to the Peritzim who abuse their wives”

[5] Shut Haran 48; Mordechai Kesubos 291 in name of Rav Avigdor, brought in Darkei Moshe 1:12; Levush 1:10; Beis Shmuel 1:21

[6] Maharam Shick E.H. 4, based on Yevamos 120a

[7] Sheilas Yaavetz 2:15

[8] Mishkanos Yaakov 1; Shoel Umeishiv Kama 1:178

[9] Shemen Sasson 34 in name of Maharam Padvah

[10] Bava Basra 60b

[11] Beis Shmuel 1:21; Darkei Moshe 1:11 in name of Mordechai Kesubos 291

[12] Mishkanos Yaakov E.H. 1, brought in Pischeiy Teshuvah 1:19

[13] Beis Shmuel 1:21