Before us is Mishnah 9 in chapter 7 of Maseches Kesubos. Until now we dealt with defects found in the wife, and now the Mishnah comes to discuss defects that developed in the husband, and the question of whether he is compelled to divorce her.
The wording of the Mishnah:
"Ha'ish shenoldu bo mumin - ein kofin oso lehotzi" - a man in whom defects developed: beis din does not compel him to give his wife a get.
The words of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel qualifies this and asks: in what case is it said that he is not compelled to give a get? And he distinguishes between two types of defects:
Minor defects: defects that the wife is expected to be able to bear and cope with - he is not compelled to divorce her.
Major defects: as the Gemara details - for example, if he became blind in one eye, his hand was cut off or his leg was broken, defects that she may not be able to cope with - he is compelled to divorce her and give her a get if that is what she wants.
The Gemara's addition:
The Gemara explains that Rabban Shimon ben Gamliel holds this way even in a case where she married him knowing that he had those major defects, and she thought she would be able to manage with them. If afterward she realized that she cannot bear it, and she said: I thought I could accept it, and now I cannot - even then she can demand the divorce, and he must give her her kesubah.
In summary: in this Mishnah we learned that the rule is that a husband in whom defects developed is not compelled to divorce his wife, while Rabban Shimon ben Gamliel distinguishes between minor defects, for which he is not compelled, and major defects, for which he is compelled to divorce her - and this applies even when she married him from the outset knowing about the defects, and afterward realized that she cannot bear them.