Parashas Naso
וְשָׁכַ֨ב אִ֣ישׁ אֹתָהּ֮ שִׁכְבַת־זֶ֒רַע֒ וְנֶעְלַם֙ מֵעֵינֵ֣י אִישָׁ֔הּ וְנִסְתְּרָ֖ה וְהִ֣יא נִטְמָ֑אָה וְעֵד֙ אֵ֣ין בָּ֔הּ וְהִ֖וא לֹ֥א נִתְפָּֽשָׂה׃
in that another man has had carnal relations with her unbeknown to her husband, and she has kept secret the fact that she defiled herself without being forced, and there is no witness against her—
Torah Temimah points to TT / Niddah / Sotah (multiple units) (verified).
ושכב איש — איש ולא קטן.
English: “And a man lay” — a man, and not a minor.
Custom
Chazal limit the relevant act to an adult male and develop the evidentiary rules of the sotah from this verse.