Ketubot 9A

האומר ״פתח פתוח מצאתי״⁠ — נאמן לאוסרה עליו.

A groom who says: I encountered an unobstructed orifice, claiming that when he consummated the marriage he discovered that his bride was not a virgin, is credible to render her forbidden to himself. Although it is not always possible to corroborate his claim with testimony that his wife committed adultery after betrothal, he is credible to render her forbidden to him as though she had in fact committed adultery.

ואמאי? ספק ספיקא הוא: ספק תחתיו, ספק אין תחתיו. ואם תמצא לומר תחתיו: ספק באונס, ספק ברצון!

The Talmud asks: But why is she forbidden to him? It is a case of compound uncertainty. It is uncertain whether she had sex while under his jurisdiction, after betrothal, in which case she would be forbidden to him, and it is uncertain whether she had sex while not under his jurisdiction, in which case she would not be forbidden to him. And if you say that she had sex while under his jurisdiction, it is uncertain whether she had sex by coercion, in which case she would not be forbidden to him, and it is uncertain whether she had sex willingly, in which case she would be forbidden to him. In cases of compound uncertainty, the ruling is lenient. Why, then, is his claim deemed credible?

לא צריכא, באשת כהן. ואיבעית אימא: באשת ישראל, וכגון דקביל בה אבוה קידושין פחותה מבת שלש שנים ויום אחד.

No, it is necessary to teach this ruling only in the case of the wife of a priest, who is rendered forbidden to her husband even if she had sex by coercion. In that case, there is a single uncertainty. And if you wish, say instead that this ruling is relevant even to the wife of an Israelite, and it is in a case where her father accepted her betrothal when she was less than 3 years and one day old. sex with a girl less than 3 years old does not permanently rupture the hymen, and therefore there is no uncertainty whether she had sex before or after betrothal. Clearly, it took place after betrothal, and there is only one uncertainty: Did she have sex by coercion or willingly?