Case:


Ruling: Regarding the opposite case, where a woman is disgusted by and unwilling to live with her husband, Rishonim dispute whether the husband can be forced to give a get (Rambam, Ishut 14:8) or not (Rabbeinu Tam in Tosafot, Ketubot 63b). The fear that she may have an interest in another man is a reason not to require the get. In this case, the man says he is disgusted by his wife, but there is specific reason to suspect that he is motivated by his interest in another woman. If he is permitted to take a second wife, we would not need to suspect that he wants the get only to remarry. We thus must see if Cherem D’rabbeinu Gershom (=CDRG- the ban on polygamy) applies to this case (note- the husband is Sephardic, upon whom CDRG is not normally binding).
Rav Yisraeli reasoned that since the husband signed an Ashkenazi ketuba at his wedding, he is bound to Ashkenazic marriage-related commitments, including CDRG. Although he might have been unaware of the differences between ketubot, his acceptance of the ketuba binds him. It is similar to what the Shulchan Aruch (Choshen Mishpat 45:3) rules: "If one admitted with his signature and the document was written in the script of non-Jews, it is clear that he did not know how to read it and there are witnesses that he signed without reading it, he is still obligated by everything that is written in it."
The Rashba (Shut I, 1156) deals with a ketuba with added clauses, based on which a woman should lose a dispute regarding monetary rights. The Rashba says that the ketuba does not cause her to lose those rights despite her agreement to accept it, citing two reasons: 1) It is the husband, not the wife, who signed the ketuba; 2) All know that the woman does not know what is included in the ketuba. The first reason does not apply to our case because the husband signed the ketuba. The second reason also does not apply because there the ketuba and the loss of money are only indirectly connected; in our case, it is direct. Furthermore, the overwhelming consensus of those who get married in our country [including Sephardim] is to not take a second wife. Therefore, we assume that he committed not to take a second wife and cannot force a get on his wife.
Rav Kapach ruled that the ketuba does not change the husband’s natural minhag, as few understand differences between ketubot, especially in regard to CDRG, which it does not explicitly address and depends on the different edot’s customs. Therefore, the husband is treated according to Sephardic custom despite his, apparently inadvertent, signing on an Ashkenazi ketuba.