QUESTION: Rav Yosef rules that when an Arusah is found to be pregnant and both she and the Arus (groom) claim that the Arus is the father, Beis Din accepts her word and does not assume that she committed Z'nus. She is permitted to marry a Kohen (such as when the Arus is a Kohen) and the child is not Pasul. Rav Yosef says that there are two reasons for this ruling. The first is that the Arus agrees with her claim. The second is that the Halachah follows the view of Raban Gamliel who says that the woman is believed when she makes a claim of certainty ("Ta'anas Bari") that she lived with a man who is not Pasul (in this case, her Arus).
Abaye challenges Rav Yosef and asks that Rav Yosef's second reason is not valid. Rav Yosef implies that even when the Arus does not agree that he is the father of the child, Beis Din still believes the Arusah's claim, based on Raban Gamliel's ruling that the woman is believed. Abaye asks that, on the contrary, we do not rely on Raban Gamliel's ruling in a case where there are "Rov Pesulim Etzlah," where most of the men around her will invalidate her to Kehunah (such as in this case, where having relations with any man other than her Arus will invalidate her to Kehunah). Rav Yosef answers that in a situation of b'Di'eved, the Halachah is like Raban Gamliel even in a case of "Rov Pesulim." Here, it is a situation of b'Di'eved because the woman is already betrothed to her Arus and wants to marry him, and thus we do not prohibit her to him.
What is the meaning of Rav Yosef's response? Raban Gamliel permits the woman in the case of the Mishnah only because the husband's claim does not contradict the woman's claim; the woman's claim is a "Ta'anas Bari" and her husband's claim is a "Ta'anas Shema" (a claim of uncertainty), and "Bari" prevails over "Shema." When, however, the Arus does not agree that he is the father and the Arusah claims that he is, their claims contradict each other (both are claims of certainty) and she should not be believed!
(a) TOSFOS, the TOSFOS HA'ROSH, and other Rishonim answer that the case Rav Yosef and Abaye argue about (where the Arus does not agree that he is the father) is not a case where the Arus denies being the father, but rather where the Arus is not available to testify (for example, he died or is abroad), and therefore Beis Din cannot ask him for his view and does not know what he would claim. That is why the case is one of "Bari v'Shema," where Raban Gamliel says that she (with her claim of "Bari") is believed.
(b) The RI (cited by Tosfos) explains that according to Abaye in Yevamos (69b), when an Arusah is suspected of being promiscuous with her Arus she is also suspected of being promiscuous with other men. Rav Yosef here is saying that according to this logic the confession of the Arus is meaningless, because once we know that she lived with her Arus (as she admits) we must assume that she lived with other men as well. Therefore, regardless of what the Arus claims, his claim is always treated like a "Shema" (and her claim is a "Bari"), and that is why Raban Gamliel permits her in this case.