Yevamos, Chapter 8, Mishnah 3. The previous two mishnayos dealt with the way in which an Ammonite and a Moabite are forbidden to enter the congregation of Israel. Our mishnah broadens the scope and discusses additional prohibitions, further groups that do not enter the congregation.
An Ammonite and a Moabite:
"Amoni u'Moavi asurim, v'issuran issur olam" - a member of the nation of Ammon or of the nation of Moav, who are the descendants of Lot's daughters, is forbidden to enter the congregation of Israel even after he converts. This prohibition is a permanent one and continues forever, as long as he is identified as a descendant of Ammon and Moav.
"Aval nekeivoseihem mutaros miyad" - an Ammonite woman and a Moabite woman are permitted immediately. This is of course the famous case of Ruth the Moabite, who was permitted to enter the congregation. The Gemara relates that already at that time there was a major dispute about it: Boaz married Ruth because he held the position that the prohibition applies to the males alone and not to the females, while his relative, Ploni Almoni, refused to marry her because he held the other view, or was concerned for it. We rule clearly that a Moabite woman is permitted, and therefore King David is a fully valid Jew.
An Egyptian and an Edomite:
"Mitzri v'Edomi einam asurim ela ad shelosha doros" - a convert from the nation of Egypt or from Edom, a descendant of Esav, is forbidden for only three generations: the first generation, the one that converted, is forbidden, and their children are forbidden as well, but the grandchildren are already permitted to enter the congregation of Israel. "Echad zecharim v'echad nekeivos" - the law applies both to the males and to the females.
This is the view of the first Tanna, while Rabbi Shimon holds: "Mutaros ha'nekeivos miyad" - even regarding an Egyptian and an Edomite, whose prohibition is limited to three generations (and in practice to two generations), the prohibition applies to the males alone and not to the females.
Rabbi Shimon's kal vachomer:
Rabbi Shimon bases his position on an argument as to why the females should be permitted immediately, exactly as was stated regarding Ammon and Moav: "Amar Rabbi Shimon, kal vachomer ha'devarim: u'mah im b'makom she'asar es ha'zecharim issur olam, hitir es ha'nekeivos miyad - makom shelo asar es ha'zecharim ela ad shelosha doros, eino din she'nateir es ha'nekeivos miyad?" In plain terms: with Ammon and Moav, where the prohibition on the males is severe and continues for all generations, the Torah nevertheless permitted the females immediately; with an Egyptian and an Edomite, whose law is more lenient and where the males are forbidden only until the third generation, all the more so should the females be permitted immediately.
The Sages' response:
The Sages answered him: "Im halachah nekabel, v'im la'din - yeish teshuvah". That is, if your words are based on a halachah that you received by tradition from your teachers, we will accept them; but if you are constructing a logical argument, there is a response to it.
The Gemara brings an example of such a response: regarding Ammon and Moav the Torah gave a reason for the prohibition - because they did not greet Israel with bread and water. This reason applies to the men alone, since the women were not expected to go out, and therefore the women are not included in the prohibition. In contrast, regarding an Egyptian and an Edomite no reason is given in the Torah, and so the prohibition may well apply equally to the women too.
Rabbi Shimon answered them: "Lo chi, halachah ani omer" - even if you wish to reject the kal vachomer, I am able to respond to your response; and in any case a halachah was transmitted to me by tradition, according to which the law does not apply to the women. However, the halachah does not follow Rabbi Shimon, and we rule that the law of an Egyptian and an Edomite applies to the women as well.
Mamzerim and Nesinim:
The mishnah concludes with a further law: "Mamzerim u'nesinim asurim, v'issuran issur olam, echad zecharim v'echad nekeivos". A mamzer, and likewise a nasin - from the nation of the Givonim who converted in the days of Yehoshua - are forbidden to enter the congregation of Israel, and their prohibition continues for all generations. This prohibition applies to the men and to the women alike: a mamzer and a mamzeres, a nasin and a nesinah.
In summary: in this mishnah we learned that an Ammonite and a Moabite are forbidden permanently, while their females are permitted immediately (and from here comes the permission of Ruth the Moabite); an Egyptian and an Edomite are forbidden until three generations, and according to the first Tanna this applies both to the males and to the females. Rabbi Shimon sought to permit the females immediately on the strength of a kal vachomer, and the Sages rejected the argument by distinguishing on the basis of the reason for the prohibition regarding Ammon and Moav, which applies to the men alone. As a matter of halachah we follow the first Tanna. We also learned that mamzerim and nesinim are forbidden permanently, males and females alike.