If a woman says to her husband "טמאה אני לך" - "I am forbidden to you" [because of an act of infidelity], she is not believed because we suspect that maybe she has her eyes on another man ["נתנה עיניה באחר"] and in order to get a divorce from her husband she made up a story. Therefore she is permitted to him and remains married.
FREGT the Baruch Taam, that since we say that she is permitted to him, if so we have no reason to believe that she made up a story to get a divorce. That being the case - she has nothing to gain from making up a story so she should be believed and thus אסורה! Ahh - she is believed? Then she should be מותרת to him because we once again suspect that she has her eyes on another man. But then again - she SHOULD be believed and אסורה because she has no reason to make up a story.
And we go round and round and round. 💿
The איסור brings the היתר which brings the איסור which brings the היתר.
