Parashas Naso · Passuk 5:13
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—
Chabad.org — English translation + Rashi
Source: Sotah 2b.
English: “And a man lay” — a man, and not a minor.
In context: The term “ish” excludes a minor (under nine years and a day) whose act does not constitute the bi’ah relevant to the sotah ordeal. Many further units on this dense passuk address the conditions of seclusion and evidence.
Custom
Chazal limit the relevant act to an adult male and develop the evidentiary rules of the sotah from this verse.