A priest [Kohen from the male descendants of Aaron the brother of Moses] has more restrictions on who he can marry than a regular Israeli. As you can see in the verses in Leviticus the section of "Emor" "Speak to the priests", he can not marry a "Zona" which is any woman who has had sex with someone that was forbidden to her (by a prohibition from the Torah.). It matters not if it was rape or not.
This is different than a Israeli. If his wife was raped, then she is still permitted to him. But if it was with her agreement, then she is forbidden to her husband from then on.
This is a bit of an introduction. Next between betrothal and marriage there used to be a long time period. But betrothal was done in such a way that she became a married woman. [That is called Kidushin.]
So if a Israeli marries, [that is did kidushin betrothal and much later does nisuin that is the actual bringing her into his home] and finds his wife is not a virgin, she is still permitted to him because of doubt of a doubt. ספק ספקא. Maybe the sex was before she got married [betrothal] . And even if it was after, maybe it was rape. So because a doubt of a doubt ספק ספקא is permitted, she is permitted to her husband. [The rule is in a case of a doubt about a prohibition of the Torah, the law is to forbid. But if the case is a doubt of a doubt, the law is to permit. ]
The Shita Mekubetzet asks why is this not a case of טומאה ברשות היחיד uncleanliness in a private domain-which we learn from "sota" [a married woman who has strayed] is forbidden? He answers that is when there is a prior status that there can at least begin a doubt. [Like in the case of Sota when there are reasons to believe she strayed. Here there is nothing like that.
Another answer that Rav Shach gives is this is not really a case of טומאה uncleanliness. but of prohibition. Only if we already know that she is forbidden, then it becomes a case of טומאה uncleanliness, but here we do not know in the first place if she is forbidden.
