The Torah ( 23) lists people who are not permitted to “join” the Jewish people through marriage. Aside from individuals from foreign nations (e.g. Egyptians and Midianites), the Torah also includes men who have been physically injured in a way that affects their ability to have normal sexual relations (see 23:2). In an attempt to define one of these categories, our Gemara brings the opinion of Rabbi Yishmael the son of Rabbi Yohanan ben Beroka, who says that he learned from the Sages in kerem beYavne that someone who has only one testicle does not fall into these categories and can marry freely.
From a medical perspective, a person who is in this state, whether he was born with just one testicle or if he lost it through an injury or an operation, can still produce viable sperm and have children. Thus, such a person would not fall into the category of someone who cannot marry.
The Sages of kerem beYavne were those who learned in the great yeshiva in Yavne, which was the seat of the Nasi after the destruction of the second Temple. According to the Talmud Yerushalmi, the gathering was not called kerem beYavne because of a vineyard that grew there (kerem = vineyard), but rather it was because the students sat in a series of long rows that were reminiscent of the standing vines of a vineyard. Since this was the gathering place of the majority of the Sages of that generation, it became known as the beit ha-va’ad – the gathering-place of the committee (of scholars). This is the place where the most serious issues of halakha – those that would impact on the future of the Jewish community in a particularly difficult period in history – were raised for discussion; therefore the decisions that were made in kerem beYavne were treated with the greatest respect by all.