Parashas Acharei Mos · Passuk 18:20
Chabad.org — English translation + Rashi
Torah Temima points to Yerushalmi Sotah 5:1.
English: “To be defiled through her” — the matter depends on her: if she acted deliberately she is forbidden; if unwittingly, she remains permitted.
Wife’s intent and future status
Whether the wife becomes forbidden to her husband after the affair turns on whether she was intentional or mistaken.