Rava said: The wife is permitted, for had he committed a sin, he would have been pleased that the husband should eat from it and die, as it is written: For they have committed adultery, and there is blood on their hands.
The Gemora asks: Surely that is obvious?
The Gemora answers: I might have thought that he had committed a sin, and as for his warning, that is because he prefers the husband not to die, so that his wife may be to him as stolen waters are sweet, and bread of secrecy is pleasant; therefore Rava teaches otherwise.
Tosfos explains that although it is true that “stolen waters are sweet,” an adulterer is not aware that this is the cause for his pleasure; rather, he thinks that his desires will be fulfilled if the husband dies and he can marry her publicly.
Rabbi Eliyahu Dessler in Michtav M’Eliyahu explains as follows: The reason that “stolen waters are sweet” is because the forbidden pleasures which the sinner desires are out of his reach. Once he reaches it and brings himself to the gratification that he was seeking, he loses his desire for that enjoyment.
The sinner, however, does not realize this. He thinks that he is genuinely attracted to that forbidden pleasure. He therefore deludes himself into thinking that finding a legal way to enjoy this forbidden pleasure will bring him the gratification that he desires.
Rava understood that this man did not realize what the driving force behind his desires is. This is why he ruled that the woman is permitted. If he indeed was an adulterer, he would have chosen the simple way for him to fulfill his desires; let the husband die. He did not do that, and that to Rava was an indicator that he was indeed innocent.