Nedarim 91 - Adultery
If a woman says to her husband, "I am defiled for you," - he has to divorce her and pay her the Ketubah. We are talking about the situation where a wife of a Kohen was violated. Unlike a regular Jew, the Kohen cannot remain married to her.
Similarly, if she says, "Heaven between us," - this is an euphemism to say that he is impotent - she is likewise believed and gets a divorce and Ketubah. And, a similar law applies if she says, "I am removed from all Jews." This is because she finds cohabitation painful. He cannot annul such vow, but rather has to divorce her and pay a Ketubah.
All these rulings were changed when the Sages saw that people were applying them to cheat others. Therefore, for all such claims the wife is not believed. For impotency, they do a "polite request." However, there are many ways to understand this: that a husband makes a banquet for his wife to convince her to be silent about this, or that the court makes such a request, etc.
One time a husband entered a house in which a man, known for his adventures with women, was hiding. The husband wanted to eat some cress, but the hiding man saw that a snake has tasted it, so he warned the husband. The question arose, was the wife of the man now prohibited to him because of possible adultery. Rava said that she was permitted, for had adultery really happened, the man would rather prefer to see the husband dead. Rava supported his view with the quote " They committed adultery and the blood is on their hands ."
Why did Rava need a quote? His logic seems right!? - There is an opposing idea, " Stolen waters are sweet, and the bread of secrecy is pleasant ." So perhaps the lovers would prefer occasional meetings in secret to full availability. - This is why Rava needed his proof.
Art: A snake in the grass William Oliver