Yevamot 85B

קנסו אותו כתובה. ומפני מה אמרו שניות מדברי סופרים אין להן כתובה ⁠— מפני שהוא כשר והיא כשרה. וכל מקום שהוא כשר והיא כשרה ⁠— קנסו אותה כתובה.

they penalized him through the marriage contract. In other words, the rabbis did not exempt him from payment of the marriage contract in that case. Since he is disqualified from the priesthood until he divorces her, the marriage will not last, and they did not force her to forfeit her marriage contract. And for what reason did they say that women forbidden as secondary relatives by rabbinic law do not have a marriage contract? Because he is fit for the priesthood and she is similarly fit, the couple therefore sees no need to divorce, and any place where he is fit and she is fit they penalized her by exempting him from payment of the marriage contract, in order to speed up the divorce.

רבי אומר: הללו דברי תורה ⁠— ודברי תורה אין צריכין חיזוק. והללו דברי סופרים ⁠— ודברי סופרים צריכין חיזוק. דבר אחר: זה ⁠— הוא מרגילה, וזו ⁠— היא מרגילתו.

R' Yehuda HaNasi says a different reason: These cases, a widow married to a High Priest and a divorcée married to a common priest, are prohibited by Torah law, and Torah law does not require strengthening by means of additional enactments. But those secondary relatives are forbidden by rabbinic law, and rabbinic law does require strengthening. Alternatively, the Talmud presents a second explanation: In this case, when they are both disqualified, it is he who encourages her to live with him despite the deleterious effect the prohibition will have on her and her offspring. Therefore, they penalized him by making him pay the marriage contract. But in that case, when they both remain fit for the priesthood despite the prohibited nature of their marriage, it is she who encourages him, and they consequently penalized her.