How much credibility does a woman have to testify about the identity of her lover?
The Mishna on our daf teaches that if an unmarried woman who is pregnant identifies the father as a specific person, who is a kohen, Rabban Gamliel and Rabbi Eliezer rule that we accept her word, while Rabbi Yehoshua says that we cannot trust her, and must assume that the father has a problematic family lineage, unless she brings proof of her claim.
It is clear from the rishonim that even according to Rabban Gamiel, we do not fully accept her testimony, and we are not convinced that the specific person who she points to is the father. Thus, even though she claims that the father was a kohen, we will not allow her child to eat teruma (which is permitted only to kohanim), nor will we allow the child to perform the sacrificial service in the Temple. The point of her statement as described in the Mishna is that if we believe her, it is because she is able to point to a specific individual with whom she had sexual relations. If she merely said that she is certain that the person she slept with was “kosher” (i.e. he was Jewish and not a mamzer) we would not believe her, even according to Rabban Gamliel.
The Talmud Yerushalmi explains that the argument between Rabbi Yehoshua and Rabban Gamliel is based on the particular concern that Rabbi Yehoshua has with people who engage in sexual relations outside the framework of marriage. It is specifically in such a case that Rabbi Yehoshua is concerned that the father might have a problematic family history, because such people are more likely to be engaging in zenut – sexual relationships outside of marriage. Rabban Gamliel’s argument is that when dealing with issues of sexuality, we can never be sure what went on, and the suspicion is the same for everyone. Therefore, if we accept her word, it is good enough.