Our Gemara on Amud Aleph discusses the circumstances of the Mishna on 25b. A judge who was unable to find grounds to annul a woman’s vows, which led to her getting divorced, may not marry that woman. This is to prevent appearances of improper bias, such as the judge having a secret motive to marry her, thus sabotaging her current marriage. 

It is important for judges to maintain the highest standards and be free of suspicion. Yet the Mishna also says that if the judge was married at the time, we do not suspect that he was thinking of this other woman. Should his wife subsequently die after his ruling, he may marry this woman. (There is a discussion among the poskim if we allow this when there was a known lack of sholom bayis between the judge and his first wife, as then there is grounds for suspicion. See Shulkhan Arukh, EH 12:4).

The obvious question we should ask is, since in the times of the Mishna and Talmud, polygamy was permitted, what difference does it make that the judge was married? He still could have been interested in this woman petitioner and sought to marry her after he sabotaged her first marriage? Tiferes Yisrael (Yachin) on the Mishna says, still, it was uncommon to take a second wife, so we do not suspect that he planned to marry this woman while still married to his wife.

It would seem that prior to the Cherem of Rabbenu Gershom there already was reluctance toward polygamy. On a simple level, it is common sense that it is not a formula for peace in the home. The very Hebrew term for co-wife, is Tzara, which literally translates as “pain from constriction”, and we politely translate as “rival”. Additionally, given that the population ratio of men to women is approximately 50/50, it is greedy to take two wives, as someone will be left without a chair when the music stops. Even in our times, where there is no polygamy, sociologists discuss the phenomenon of “serial polygamy”. That is, wealthy, powerful men divorce their first wives, and the “upgrade” to a “younger model”. In effect, they are using their wealth and power to access a bride that could have married a peer. It is serial polygamy.

On a halakhic level, polygamy was not all fun and games. The husband would be required to meet the financial and sexual responsibilities to each wife and not ignore one over the other. The Gemara in Sanhedrin (107a) tells us of the great “pain” Dovid Hamelech endured to meet the responsibilities of providing adequate sexual attention to his 18 wives!

Some of this sounds strange to our ideas and sensibilities. However, the main message is ultimately about responsibility and respect, even in a culture where polygamy occurred. Rabbinic legislation is often not about adding a “new” law. Rather it is about formalizing into law an already existing moral imperative that was left to people to decide, until the rabbis felt tighter legislation was required. Before there was a decree of muktzeh, you can be sure that Moshe Rabbenu didn’t absentmindedly carry money or a pen on shabbos. Intuitively people knew the sanctity of shabbos and withdrew from certain behaviors and speech. It was not originally necessary to enact this sensibility into a fixed law. It only became necessary to legalize the moral sentiment when the rabbis saw that the people were losing that sensitivity and couldn’t properly discern the sanctity of shabbos without external boundaries and prompts. See our discussion in Psychology of the Daf Yevamos 21.

So too, prior to Cherem Rabbenu Gershom, there was a recognition of the sanctity of marriage and the usually problematic results from polygamy. In Rabbenu Gershom’s time it became necessary to legislate that which was once deeply understood.