I have a question on the Raavad. In Laws of forbidden relations [chapter 1. law 22] the Raavad says the reason a priest gets lashes in a case of warning and then her being alone with the other person (kinui and stira) and one witness is because of "and she is unclean".   

[Unclean is written  by a sota -a married woman who has been warned not to be alone with so and so and then in fact goes and is alone with so and so. ]

Rav Shach explain it thus. Normally "and she is unclean" means it is like the forbidden relations of Leviticus 18 [arayot] which we learn from Yevamot page 11 about a Sota. But a priest is different. If his wife has had sex with someone even by rape, she still is forbidden to her husband and so it is an isur [prohibition] priesthood (khuna) not arayot [forbidden relations which requires two witnesses]. And so one witness is enough to get her husband lashes since it is  a regular law and one witness is believed in regular prohibition that are not arayot.

[To the Raavad she is not a zona [a woman who is forbidden to a priest because she has sex with someone forbidden to her] because we do not believe one witness in teh case of  married woman.]

The question I have here is that the gemara in Yevmot makes no distinction between a kohen and a Israeli when it comes to an isur Sota which is the isur of "and she is unclean". I mean to say that it say a Sota does not get yibum because uncleanliness is written by her and by that there is a gezera Shava to arayot.. So we see openly the Gemara makes not distinction between whether she is the wife of a  kohen or mot. It is all Arayot and if it is all aryot you need two witness!

[I can not tell if my question is on the Raavad or Rav Shach or both.]

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A further question here is  that I am not sure I understand the answer of Rav Shach in explaining the Raavad at all. The idea that since והיא נטמאה is different for a kohen than for a Israeli therefore it is an isur kehuna. Well in what way is it different? If the sex was rape then she is forbidden to her husband because of Zona. But that is the very thing the Raavad is holding no one would get lashes for since we do not believe one witness in the case of a married woman for the isur of Zenut. If the sex was willingly, then it is the same prohibition for a Israeli and a kohen.