Talmud for Yevamot 8:3
עַמּוֹנִי וּמוֹאָבִי, אֲסוּרִים, וְאִסּוּרָן אִסּוּר עוֹלָם, אֲבָל נְקֵבוֹתֵיהֶם מֻתָּרוֹת מִיָּד. מִצְרִי וַאֲדוֹמִי אֵינָם אֲסוּרִים אֶלָּא עַד שְׁלֹשָׁה דוֹרוֹת, אֶחָד זְכָרִים וְאֶחָד נְקֵבוֹת. רַבִּי שִׁמְעוֹן מַתִּיר אֶת הַנְּקֵבוֹת מִיָּד. אָמַר רַבִּי שִׁמְעוֹן, קַל וָחֹמֶר הַדְּבָרִים, וּמָה אִם בִּמְקוֹם שֶׁאָסַר אֶת הַזְּכָרִים אִסּוּר עוֹלָם, הִתִּיר אֶת הַנְּקֵבוֹת מִיָּד, מְקוֹם שֶׁלֹּא אָסַר אֶת הַזְּכָרִים אֶלָּא עַד שְׁלֹשָׁה דוֹרוֹת, אֵינוֹ דִין שֶׁנַּתִּיר אֶת הַנְּקֵבוֹת מִיָּד. אָמְרוּ לוֹ, אִם הֲלָכָה נְקַבֵּל, וְאִם לַדִּין, יֵשׁ תְּשׁוּבָה. אָמַר לָהֶם, לֹא כִי, הֲלָכָה אֲנִי אוֹמֵר. מַמְזֵרִין וּנְתִינִין, אֲסוּרִין, וְאִסּוּרָן אִסּוּר עוֹלָם, אֶחָד זְכָרִים, וְאֶחָד נְקֵבוֹת:
An Ammonite and a Moavite are forbidden (to enter the congregation of the L rd), and their prohibition is perpetual; but their women are permitted immediately. An Egyptian and an Edomite are forbidden only until three generations, both the males and the females. R. Shimon permits the females immediately. R. Shimon said: This can be deduced a fortiori, viz.: Now if in a place where the males are interdicted perpetually, the females are permitted immediately — in a place where the males are interdicted only until three generations, should it not follow that the females be permitted immediately! They said to him: If this is the halachah, we shall accept it; and if (only) an a fortiori argument, it can be refuted. [If you are expounding an a fortiori argument of your own, it can be refuted, viz.: In the instance of Ammon and Moav, the reason (that only men and not women are interdicted) is given, viz. (Deuteronomy 23:4): "Because they did not greet you with bread and with water" — and it is not the way of a woman to greet, whereas no reason is given for (the interdict against) an Egyptian and an Edomite.] He answered: No, I am stating a halachah. Mamzerin and Nethinin are interdicted, and their interdict is perpetual, both males and females. [Nethinim are the Giveonites who converted to Judaism in the days of Joshua, and who were "given" (netunim) to be hewers of wood and drawers of water.]