Ketubot, Chapter Seven, Mishnah Nine
Up until now the mishnah has been discussing physical flaws found in a wife, which cause the marriage to either be annulled or her husband to be allowed to divorce her without paying the ketubah. Just in case you thought that men were physically perfect, the last two mishnayoth of our chapter discuss physical flaws found in a man, that force him to divorce his wife and pay her the ketubah.
A man in whom defects have arisen [after marriage] cannot be forced to divorce [his wife].
Rabban Shimon ben Gamaliel said: To what does this apply: to minor defects, but with regard to major defects he can be forced to divorce her.
If a woman marries a man and she finds that he has physical flaws that existed before the marriage and that he did not tell her about them, she can compel him to divorce her and pay her the ketubah. If she knew about them (or they were clearly noticeable and therefore should have known about them), and she married him in any case, she cannot later complain about them and use them as grounds for divorce. Furthermore, if new defects arise after the marriage, she cannot force him to divorce her.
Rabban Shimon ben Gamaliel says that the above is only true if the defect is small. If the defect which arises after marriage is large, he must divorce her. According to the Talmuds explanation of Rabban Shimon ben Gamaliel, if the defects are significant, she can force a divorce even if she knew about them before the marriage, for she can say that she thought that they wouldnt disturb her but now they do.