Part I: Midrash and Theoreticals
מָרְדְּכַי זָן וּפִרְנֵס, אָמַר רַבִּי יוּדָן פַּעַם אַחַת חִזֵּר עַל כָּל הַמֵּנִיקוֹת וְלֹא מָצָא לְאֶסְתֵּר לְאַלְתָּר מֵינִיקָה, וְהָיָה מֵינִיקָהּ הוּא, רַבִּי בֶּרֶכְיָה וְרַבִּי אַבָּהוּ בְּשֵׁם רַבִּי אֱלִיעֶזֶר בָּא לוֹ חָלָב וְהָיָה מֵינִיקָהּ. כַּד דָּרַשׁ רַבִּי אַבָּהוּ בְּצִבּוּרָא גָּחוֹךְ צִבּוּרָא לְקָלֵיהּ, אֲמַר לְהוֹן וְלָא מַתְנִיתָּא הִיא, רַבִּי שִׁמְעוֹן בֶּן אֶלְעָזָר אוֹמֵר חָלָב הַזָּכָר טָהוֹר.
Mordekhai fed and sustained others. Rabbi Yudan said: One time he went around to all the wet nurses, but did not immediately find a wet nurse for Esther, and he nursed her himself. Rabbi Berekhya and Rabbi Abahu said in the name of Rabbi Eliezer: Milk came into him [his breasts] and he nursed her. When Rabbi Abahu expounded this in public, the audience laughed as he spoke [of a man producing milk]. He said to them: But is it not [stated in] a mishna: Rabbi Shimon ben Elazar says: The milk of a male is not subject to ritual impurity.*
* From Mishna Makhshirin 6:7
אָמַר רַבִּי יִרְמְיָה בֶּן אֶלְעָזָר בְּשָׁעָה שֶׁבָּרָא הַקָּדוֹשׁ בָּרוּךְ הוּא אֶת אָדָם הָרִאשׁוֹן, אַנְדְּרוֹגִינוֹס בְּרָאוֹ, הֲדָא הוּא דִכְתִיב (בראשית ה, ב): זָכָר וּנְקֵבָה בְּרָאָם. אָמַר רַבִּי שְׁמוּאֵל בַּר נַחְמָן, בְּשָׁעָה שֶׁבָּרָא הַקָּדוֹשׁ בָּרוּךְ הוּא אֶת אָדָם הָרִאשׁוֹן, דְּיוּ פַּרְצוּפִים בְּרָאוֹ, וְנִסְּרוֹ וַעֲשָׂאוֹ גַּבִּים, גַּב לְכָאן וְגַב לְכָאן. אֲתִיבוּן לֵיהּ וְהָכְתִיב (בראשית ב, כא): וַיִּקַּח אַחַת מִצַּלְעֹתָיו, אֲמַר לְהוֹן מִתְּרֵין סִטְרוֹהִי, הֵיךְ מָה דְאַתְּ אָמַר (שמות כו, כ): וּלְצֶלַע הַמִּשְׁכָּן, דִּמְתַרְגְּמִינַן וְלִסְטַר מַשְׁכְּנָא וגו'.
Rabbi Yirmeya ben Elazar said: When the Holy One blessed be He created Adam the first man, He created him androgynous. That is what is written: “He created them male and female” (Genesis 5:2). Rabbi Shmuel bar Naḥman said: When the Holy One blessed be He created Adam the first man, He created him with two faces [That is, two conjoined bodies, male and female] and [subsequently] He sawed him in two and made [for] him two backs, a back here and a back there. They raised an objection to him: But is it not written: “He took one of his ribs [tzalotav] … [and the Lord God built the rib that He took from the man into a woman]”? (Genesis 2:21–22). He said to them: [It means that He took] one of his two sides, as it says: “And for the tzela of the Tabernacle” (Exodus 26:20), which we translate: “And for the side of the Tabernacle...”.
Part II: Law
(ה) לֹא־יִהְיֶ֤ה כְלִי־גֶ֙בֶר֙ עַל־אִשָּׁ֔ה וְלֹא־יִלְבַּ֥שׁ גֶּ֖בֶר שִׂמְלַ֣ת אִשָּׁ֑ה כִּ֧י תוֹעֲבַ֛ת ה׳ אֱלֹקֶ֖יךָ כׇּל־עֹ֥שֵׂה אֵֽלֶּה׃ {פ}
(5) A woman must not put on man’s apparel, nor shall a man wear woman’s clothing; for whoever does these things is abhorrent to your God ה׳.
(ח) מותר לישא אשה בפורים: מה שנהגו ללבוש פרצופים בפורים וגבר לובש שמלת אשה ואשה כלי גבר אין איסור בדבר מאחר שאין מכוונין אלא לשמחה בעלמא וכן בלבישת כלאים דרבנן וי"א דאסור אבל המנהג כסברא הראשונה וכן בני אדם החוטפים זה מזה דרך שמחה אין בזה משום לא תגזול ונהגו כך ובלבד שלא יעשה דבר שלא כהוגן ע"פ טובי העיר:
(8) One may marry a woman on Purim. The custom of wearing masks on Purim, and of crossdressing, is totally permitted because of its innocent and joyful purpose, as is the wearing of shaatnez [mixed fabrics]. While some would prohibit it, our practice is as I have already said. So too those who playfully rob each other do not violate "Thou shalt not steal'', and such is our custom. However, you may do only as local elders permit.
לְיַרְקוֹנָא תְּרֵין בְּשִׁיכְרָא, וּמִיעֲקַר. וּמִי שְׁרֵי? וְהָתַנְיָא, מִנַּיִין לַסֵּירוּס בָּאָדָם שֶׁהוּא אָסוּר — תַּלְמוּד לוֹמַר: ״וּבְאַרְצְכֶם לֹא תַעֲשׂוּ״ — בָּכֶם לֹא תַעֲשׂוּ, דִּבְרֵי רַבִּי חֲנִינָא. הָנֵי מִילֵּי — הֵיכָא דְּקָא מִיכַּוֵּין, הָכָא — מֵעַצְמוֹ הוּא. דְּאָמַר רַבִּי יוֹחָנָן: הָרוֹצֶה שֶׁיְּסָרֵס תַּרְנְגוֹל — יִטּוֹל כַּרְבׇּלְתּוֹ, וּמִסְתָּרֵס מֵאֵלָיו. וְהָאָמַר רַב אָשֵׁי: רָמוּת רוּחָא הוּא דִּנְקִיטָא לֵיהּ. אֶלָּא בְּסָרִיס.
The Gemara asks: And is it permitted to cause sterility? Wasn’t it taught in a baraita: From where is it derived that castration of a man is prohibited? The verse states: “Those whose testicles are bruised, or crushed, or torn, or cut, shall not be offered to the Lord, and you shall not do this in your land” (Leviticus 22:24), meaning that you shall not do it to yourselves; this is the statement of Rabbi Ḥanina. Apparently, it is prohibited to castrate a man. The Gemara answers: This prohibition applies only in a case where one intends to castrate. Here, in the cure for jaundice, the sterility happens on its own, incidental to the treatment. Proof is cited from that which Rabbi Yoḥanan said: One who seeks to castrate a rooster should remove its comb and it will become castrated on its own. Incidental castration is permitted. The Gemara rejects the proof. Didn’t Rav Ashi say: It is arrogance that it assumes when it has its comb, and when the comb is removed it becomes depressed and no longer procreates. However, it is not actually castrated. Rather, apparently this remedy for jaundice is permitted only for one who is a saris and for whom causing sterility is not a concern.