Our Gemara on Amud Aleph discusses liabilities between the upper floor residents and the lower floor residents of a home:
הָנְהוּ בֵּי תְרֵי דַּהֲווֹ דָּיְירִי, חַד עִילַּאי וְחַד תַּתַּאי. אִיפְּחִית מַעֲזִיבָה. כִּי מָשֵׁי מַיָּא, עִילַּאי אָזְלִי וּמַזְּקִי לְתַתַּאי. מִי מְתַקֵּן? רַבִּי חִיָּיא בַּר אַבָּא אָמַר: הָעֶלְיוֹן מְתַקֵּן, וְרַבִּי אִלְעַי מִשּׁוּם רַבִּי חִיָּיא בְּרַבִּי יוֹסֵי אָמַר: הַתַּחְתּוֹן מְתַקֵּן. וְסִימָן: ״וְיוֹסֵף הוּרַד מִצְרָיְמָה״.
The Gemara relates: An incident occurred with these two people who were residing in the same house, one in the upper story, and the other one in the lower story. The plaster of the floor of the upper story broke, so that when the resident of the upper apartment would wash with water, it would run down and cause damage to the lower story. The question was: Who must repair the ceiling? Rabbi Ḥiyya bar Abba says: The upper resident repairs it, and Rabbi Elai says in the name of Rabbi Ḥiyya, son of Rabbi Yosei: The lower resident repairs it.
Ben Yehoyada sees a moral metaphor in these seemingly dry halachic passages. The leak represents the toxic forces of sin, and the upper story is the power of speech (mouth, top of body), and the sexual organs are the lower floor. There is a tradition from the rabbis that sinful speech leads to sinful sexual behavior, and the Mishna is alluding to this.
Tiferes Shlomo (Purim 17) finds a clever hint for this in the verse from Megillas Esther (1:22):
וַיִּשְׁלַ֤ח סְפָרִים֙ אֶל־כׇּל־מְדִינ֣וֹת הַמֶּ֔לֶךְ אֶל־מְדִינָ֤ה וּמְדִינָה֙ כִּכְתָבָ֔הּ וְאֶל־עַ֥ם וָעָ֖ם כִּלְשׁוֹנ֑וֹ לִהְי֤וֹת כׇּל־אִישׁ֙ שֹׂרֵ֣ר בְּבֵית֔וֹ וּמְדַבֵּ֖ר כִּלְשׁ֥וֹן עַמּֽוֹ׃
Dispatches were sent to all the provinces of the king, to every province in its own script and to every nation in its own language, that every man should wield authority in his home and speak the language of his own people.
How can one “wield authority in his home”, i.e. master his bodily lusts? He must “speak the language of his own people”, i.e. master his speech.
As to why the sages considered sin with the mouth as a precursor to sexual immorality, on a simple level we may say that lack of boundaries and respect will lead to also breaking barriers of dignity and body integrity. Additionally, I suggest the following:
Both of these parts of the body have been described in the scripture as potentially being obstructed by a barrier, the Hebrew term “Arel”, which is used to describe an uncircumcised state of the body, with the foreskin considered a spiritual and physical occlusion. Actually, we find four uses of the root “A-R-L” in the Torah:
- Moshe describes his lips as “blocked” “arel sefasayim” (Shemos 6:12).
- The Torah also describes the unrepentant heart as “arel” (Devarim 10:16),
- The produce of the early years of a fruit bearing tree (Vayikra 19:23)
- And of course the actual foreskin “arlah” (Bereishis 17:11). I am not sure if the original Hebrew term “A-R-L” is foreskin and then the borrowed metaphorical term means any spiritual block, or the other way around, that “A-R-L” is a generic barrier of any sort and it is metaphorically used for the foreskin.
These zones are on a continuum, with each one a further degree of physical action and effect. It starts with thought (heart), continues to speech (lips), then the production of vegetative matter (trees), and finally the human procreative area, which is the ultimate physical act as it produces another human being. If one corrupts the heart, further physical effects will manifest, eventually leading to an undermining of the most powerful form of human physical activity. Human behavior is not accidental. Sinful or sacred activities come from precursors in thought, speech, and eventually action.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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