QUESTION: The Mishnah states that two women who were captured (Shevuyos) may testify that the other was not defiled and is permitted to marry a Kohen. RASHI writes that the Chachamim were lenient in the case of a Shevuyah to accept the testimony of one witness, even a woman.
Why does Rashi say that the Chachamim were lenient in the case of a Shevuyah? There is a general rule that "one witness is believed in matters of prohibitions" ("Ed Echad Ne'eman b'Isurim"), even when the witness is not valid for testimony in court (such as a woman or a relative). The case of Shevuyah involves a question of Isur (whether or not the woman is permitted to marry a Kohen), and thus the normal rule of "Ed Echad Ne'eman b'Isurim" should permit us to accept the testimony of a single witness! Why does Rashi say that there is a special leniency in the case of Shevuyah?
(a) The TOSFOS RID in Kidushin (66a, cited by the SHEV SHEMAITSA 6:15) proves from the Gemara there (see Shev Shemaitsa) that judging whether or not a woman is a Shevuyah, or whether she is prohibited from marrying a Kohen for another reason (such as Gerushah or Chalutzah), is considered a "Davar sheb'Ervah" and therefore two witnesses are required. Accordingly, it is clear why a special leniency is needed in the case of Shevuyah to accept the testimony of a single witness.
The source for the requirement of two witnesses in the case of a "Davar sheb'Ervah" is the verse which discusses a woman who becomes prohibited to her husband because she committed adultery (Devarim 24:1). According to the Tosfos Rid, the verse apparently refers not only to an adulteress (who transgressed an Isur Kares) but to any testimony that causes a woman to become prohibited (or permitted) to certain men. According to this logic, two witnesses should be required even to prove that a person is, or is not, a Mamzer or Mamzeres. (One might distinguish, however, between Isurim that can take effect only upon a woman -- such as Pesulei Kehunah and Z'nus, which require two witnesses -- and those that can apply to men as well -- such as Mamzer, which is not a "Davar sheb'Ervah" and requires only a single witness.)
(b) REBBI AKIVA EIGER and the NESIVOS HA'MISHPAT (both cited in TESHUVOS REBBI AKIVA EIGER #124, 125) posit based on the MORDECHAI in Yevamos that a court case is considered to be a "Davar sheb'Ervah" only if it creates (or removes) an Isur on the woman which prevents the enactment of Kidushin with that woman (similar to the Isur of Eshes Ish).
This explanation of the Mordechai needs clarification. The Isur of an adulteress to her husband does not preclude "Tefisas Kidushin"; it is simply an Isur Lav. Similarly, the Isur of Shevuyah to a Kohen is only a Lo Ta'aseh (a normal negative commandment) which does not prevent Kidushin from taking effect, but it is clear from the Gemara here and in Kidushin (ibid.) that two witnesses are required to prohibit a Shevuyah.
The Nesivos ha'Mishpat explains that according to the Mordechai there are two acts which are considered "Davar sheb'Ervah": one is an act which creates an Isur which precludes the enactment of Kidushin, and the other is an act in which a woman becomes prohibited to others because she had relations with a man with whom Kidushin could not take effect. A Shevuyah (who is suspected of being raped by her non-Jewish captors) and an adulteress fit into the second category.
However, Rebbi Akiva Eiger asserts that this is not the Mordechai's intention. He suggests instead that prohibiting an adulteress to her husband is considered a "Davar sheb'Ervah" even though it does not prevent Kidushin from taking effect with her, because the Torah refers to that particular Isur as "Tum'ah" (see Yevamos 11a) just as it refers to the Isurim of Arayos. Shevuyah is considered a "Davar sheb'Ervah" for the same reason; the Gemara in Yevamos (56b) teaches that the wife of a Kohen who was raped is prohibited to her husband because of an Isur of "Tum'ah." However, this is true only in the case of a married woman who was raped (see Teshuvos Rebbi Akiva Eiger #125, and TESHUVOS V'CHIDUSHEI REBBI AKIVA EIGER 20:3). If a single woman is captured and raped, testimony about her would not be considered a "Davar sheb'Ervah" at all according to this logic. According to Rebbi Akiva Eiger, there must be another reason for why any Shevuyah, even a single woman, would have required two witnesses without the special leniency.
This also appears to be the opinion of the RAMBAM (Hilchos Sanhedrin 16:6, see Shev Shemaitsa ibid.), who writes that only one witness is necessary to determine whether or not a woman is a Zonah or Gerushah and prohibited to a Kohen. Similarly, TOSFOS in Gitin (2b, DH Midi) writes that the law that a "Davar sheb'Ervah" requires two witnesses applies only to testimony concerning marriage, divorce, and Z'nus that prohibits a woman to her husband.
(c) TOSFOS (Gitin 2b, DH Ed Echad) writes that one witness is not believed even in order to remove an ordinary Isur that has already been established ("Ischazek Isura," or a "Chezkas Isur"). Accordingly, since the Chachamim assume that a Shevuyah definitely was defiled until proof is brought otherwise (see Kesuvos 13b), she is considered to have a "Chezkas Isur" and thus a single witness would not be believed to remove that Isur if not for the special leniency the Chachamim instituted in the case of a Shevuyah.