Explanation: Beis Shamai's reason is like we explain below according to R. Yehudah - do we burn the entire house?!

Question: Why do we need a verse for Mezid? What would be the source to forbid?

Suggestion: We would learn from the Kal va'Chomer above. One who had Bi'ah with a light Isur (Eshes Ish), the one who forbids her (her husband) becomes forbidden...

Rejection: Since he holds like that Kal va'Chomer, why does R. Yosi need a different Kal va'Chomer here? He should have argued with R. Yehudah simply. This Kal va'Chomer is not relevant to their argument at all!

Explanation: I.e. this Isur can come through anyone. One's divorcee is forbidden through anyone who marries her;

However, Achos Ishah, even if one's wife becomes forbidden through Bi'ah with her sister, she is not forbidden through another man who has Bi'ah with her.

Explanation: Now we explain according to the opinion (above, 29a) that Ma'amar acquires mid'Oraisa.

Explanation: This is unlike Achos Ishah, in which we seek to forbid her through Bi'as Isur. Here, it depends on Zikah!

Explanation: When her husband has Bi'ah with her, she is forbidden permanently to the Bo'el, for then the water will not check her.

Question: Why does it say that he had Bi'ah with her? Even if he will divorce her, and then be Mekadesh her and have Chupah with her, or say that he will not give to her to drink, he cannot give to her to drink, and she is forbidden permanently to the Bo'el! (Maharsha - one might have thought that she does not drink after divorce because she is not married to him, but if he remarries her, she can drink. Tosfos teaches that this is not so.)

Question (Rabbeinu Yitzchak): What is the source that once he says that he will not give to her to drink, he cannot [retract and] give to her to drink?

Answer (R. Yitzchak): There is somewhat of a proof, for the Mishnah (Sotah 23a) lists one who says "I will not give to her to drink" among those whose Menachos are burned, for they can never drink. (Aruch l'Ner says that he can retract and give to her to drink. If he does not retract in time, the Minchah will become Nosar, so it is burned. Surely, if he says that he will not give to her to drink because the water will not test her, i.e. he is not clean from sin, or he knows that she was Mezanah, he can never retract because he already admitted. - PF)

Implied question: Why don't we ask that one is choked [for Eshes Ish, but there is no Misah for Achos Ishto]?

Answer: We should be stringent [about Achos Ishto] because the Isur lasts the entire life [of the one who forbids her, i.e. her sister].

Question (Ri): Still, we could ask all the questions above (to show that Eshes Ish is more stringent than Achos Ishto), i.e. her body was defiled, and the Isur is through all men!

Implied question: Why don't we explain "whoever does not forbid to others" to be when Achos Ishto is single?

Answer: This is unreasonable, for "whoever does not forbid to others" implies that there are others to whom [one might have thought that he forbids, but] he does not forbid.

Question: What is his source that the first Tana forbids Giso's wife?

Perhaps even [in a case of] Arusaso v'Giso, the first Tana permits Eshes Giso, since she does not need a Get;

... Just like R. Yosi holds that when Eshes Giso does not need a Get from her second [husband], e.g. when Ishto v'Giso [went overseas], he permits Eshes Giso!

Answer (Ri): Since the first Tana discusses the Heter of his own wife, but not Eshes Giso, this shows that Eshes Giso is forbidden to Giso...

Even though she does not need a Get, and one cannot say that there was a Tenai [in her marriage, and it was not fulfilled], and [people will not say that] this one (Giso) divorced her and this one married her, in any case Chachamim fined [her].

However, surely R. Yosi fines to forbid her to return to Giso only when she needs a Get, for then it looks like Machazir Gerushaso, like the case of Arusaso v'Giso, when one can say that there was a Tenai in her Kidushin.

Observation: Below, Shmuel says that Arusas Achiv is not like Eshes Ish. He holds like Rabanan in one way, that people do not attribute that there was a Tenai;

He holds like R. Yosi in one way, that when she does not need a Get, she does not resemble Machazir Gerushaso, and we do not fine to forbid her.