sexuality - Is it considered a good thing that Lot's daughters slept with their father?

In Bereishis 19:30-38 the story is told how the 2 daughters of Lot thought (for whatever reason) that they should sleep with their father. From the psukim themselves, it is unclear whether or not what they did is a positive thing. Are there sources that make it clear how to relate to what they did?

Answer

There is a certain amount of ambiguity here.

On the one hand, we see that our Sages praise the daughters of Lot, and especially the elder daughter who took the initiative.

Nazir 23a:

אלא משל ללוט ושתי בנותיו עמו הן שנתכוונו לשם מצוה וצדיקים ילכו בם הוא שנתכוין לשם עבירה ופושעים יכשלו בם

Rather, it is comparable to Lot and his two daughters, who were with him. They, who intended to engage in sexual intercourse with him for the sake of a mitzva, as they thought that the entire world was destroyed and wished to preserve the human race, are described in the first part of the verse: “And the just walk in them.” He who intended to act for the sake of a transgression is described by the last part: “But transgressors stumble over them.”

(Translation and elucidation courtesy of sefaria.org)

Nazir 23b:

א"ר חייא בר אבין א"ר יהושע בן קרחה לעולם יקדים אדם לדבר מצוה שבשכר לילה אחת שקדמתה בכירה לצעירה זכתה וקדמה ארבעה דורות בישראל למלכו':‏

Rabbi Ḥiyya bar Avin said that Rabbi Yehoshua ben Korḥa said: A person should always come first with regard to a matter of a mitzva, as in reward of the one night that the elder daughter of Lot preceded the younger for the sake of a mitzva, she merited to precede the younger daughter by four generations to the monarchy of the Jewish people. The descendants of Ruth the Moabite ruled over the Jewish people for four generations: Obed, Yishai, David, and Solomon, before the reign of Solomon’s son Rehoboam, whose mother was Naamah the Ammonite.

(Translation and elucidation courtesy of sefaria.org)

On the other hand, our Sages castigate them for their lack of shame, especially the elder who was more brazen.

Bamidbar Rabbah 20:

וַיְהִי מִמָּחֳרָת וַתֹּאמֶר הַבְּכִירָה אֶל הַצְּעִירָה הֵן שָׁכַבְתִּי אֱמֶשׁ, לִמְּדַתָּה אֲחוֹתָהּ, וּלְפִיכָךְ חָסַךְ הַכָּתוּב עַל הַצְּעִירָה וְלֹא פֵּרְשָׁהּ, אֶלָּא (בראשית יט, לה): וַתִּשְׁכַּב עִמּוֹ, וּבַגְּדוֹלָה כְּתִיב (בראשית יט, לג): וַתִּשְׁכַּב אֶת אָבִיהָ.‏

"And it was the next day and the elder said to the younger, behold last night I slept etc." She taught her sister, therefore the verse took pity on the younger daughter and was not explicit, rather writing, "And she slept with him", as opposed to the older daughter, where it writes, "And she slept with her father."

Nazir 23b:

א"ר חייא בר אבא א"ר יוחנן מנין שאין הקב"ה מקפח אפי' שכר שיחה נאה דאילו בכירה דקריתיה מואב א"ל רחמנא (דברים ב, ט) אל תצר את מואב ואל תתגר בם מלחמה מלחמה הוא דלא אבל צעורי צערינן ואילו צעירה דקריתיה בן עמי אמר ליה (דברים ב, יט) אל תצורם ואל תתגר בם אפילו צעורי לא תצערינן כלל

Rabbi Ḥiyya bar Abba said that Rabbi Yoḥanan said: From where is it derived that the Holy One, Blessed be He, does not deprive one of even the reward for proper speech, i.e., for speaking in a refined manner? As while there is the case of Lot’s elder daughter, who called her son Moab [mo’av], which alludes to his shameful origins, as me’av means: From father, and the Merciful One says to Moses: “Do not besiege Moab, nor contend with them in war” (Deuteronomy 2:9), which indicates: It is war that is not permitted; however, with regard to harassing, the Jews were permitted to harass them. And while there is the case of Lot’s younger daughter, who called her son Ben-Ami, son of my people, without explicitly mentioning her father. With regard to her descendants, God said to Moses: “Do not harass them, nor contend with them” (Deuteronomy 2:19), which means even as far as harassing is concerned, you may not harass them at all.

(Translation and elucidation courtesy of sefaria.org)