Everyone knows why everyone gathered for the wedding.
By analogy: Everyone knows why the matter is done. The matter is known and there is no reason to speak of it openly. Or: There is a need to maintain pure language (as it is known to all that the desire for married life fully brings the woman, as the man, to want to marry, yet this is not something spoken of openly).
("Rabbi Hanan bar Rava said: Everyone knows everything for which a marriage was arranged, except for anyone who has a bad mouth, even if they seal it with a seal, a judgment of seventy years for good will be passed upon him - it will be turned against him for evil" - Shabbos 33a)
Translation
Expression (Hebrew)
הַכֹּל יוֹדְעִים לָמָּה כַּלָּה נִכְנֶסֶת לַחֻפָּה
Definition (Hebrew)
בְּהַשְׁאָלָה: כֻּלָּם יוֹדְעִים לָמָּה הַדָּבָר נַעֲשֶׂה. הַדָּבָר יָדוּעַ וְאֵין סִבָּה לְדַבֵּר עַל כָּךְ בְּגָלוּי. אוֹ: יֵשׁ לְהַקְפִּיד עַל לָשׁוֹן נְקִיָּה (כְּמוֹ שֶׁיָּדוּעַ לְכֻלָּם כִּי הָרָצוֹן לְחַיֵּי זוּגִיּוּת מְלֵאִים מְבִיאִים אֶת הָאִשָּׁה, כְּמוֹ אֶת הָאִישׁ, לִרְצוֹת לְהִתְחַתֵּן, וְאוּלָם אֵין זֶה דָּבָר שֶׁמְּדַבְּרִים עַל כָּךְ בְּגָלוּי).
("אָמַר רַבִּי חָנָן בַּר רָבָא: הַכֹּל יוֹדְעִין כַּלָּה לָמָּה נִכְנְסָה לַחֻפָּה, אֶלָּא כָּל הַמְנַבֵּל פִּיו אֲפִלּוּ חוֹתְמִין עָלָיו גְּזַר דִּין שֶׁל שִׁבְעִים שָׁנָה לְטוֹבָה- הוֹפְכִין עָלָיו לְרָעָה"- שַׁבָּת לג ע"א)