Gemara

33a Question (Rami bar Chama): If semen exudes from a woman during the seven clean days, does this interrupt her count?

(Semen that exudes is Metamei even a Tahor woman.) Is she considered k'Ro'eh (like one who saw blood)?

Or, is she Teme'ah k'Noga'as (only like one who touched semen)?

Objection (Rava): Rami's question is ill-founded!

If it would interrupt her count, it could not be Soser (uproot) all the days she counted. Surely, it is not more stringent than the man from whom it came! (Semen is Soser only one day of a Zav's count.)

It cannot be Soser only one day. "V'Achar Tit'har" teaches that she must immerse after all the (seven clean) days were consecutive!

Counter-question: According to Rava, why does semen interrupt only one day of a Zav's count? There may not be any Tum'ah in the middle of the seven days, for it says "l'Taharaso"!

Answer: Rather, "l'Taharaso" must teach that there may not be Tum'as Zivah in the middle.

Answer: Likewise, "v'Achar Tit'har" teaches that Tum'as Zivah may not interrupt!

37a - Question (Rava): Does blood seen b'Koshi Soser (cancel all) the clean days already counted after Zivah?

Perhaps only blood that is Gorem (causes Tum'as Zivah ) is Soser. This blood is not Gorem!

Answer (R. Avin's father - Beraisa): Zov is Gorem (causes) a man to be Tamei for seven days, therefore it is Soser seven days.

The Beraisa specifies that Zov is Gorem Tum'ah, to teach that only what is Gorem is Soser.

41b (Beraisa - R. Shimon): If not for "(if a man has Bi'ah with a woman...) v'Rachatzu va'Mayim v'Tam'u Ad ha'Arev", we would say that the semen is not Metamei her, for it touches her internally. It is Tum'as Beis ha'Starim;

The Torah decrees that she becomes Teme'ah anyway.

R. Shimon expounds that semen during Bi'ah is Metamei her, even though it is Tum'as Beis ha'Starim. Semen that she discharges is not Metamei her until it leaves her (and touches her externally).

Question: Even before the semen is discharged, she is Teme'ah due to Bi'ah!

Answer: The case is, she immersed after Bi'ah. The discharge of semen is Metamei her again.

Inference: (Immersion helps until she discharges semen, i.e.) after Bi'ah she has only Tum'as Erev.

Question: Rava forbids a woman to eat Terumah for three days after Bi'ah, for surely she will discharge semen!

Answer #1: The Beraisa discusses a woman who was carried to the Mikveh on her bed. (As long as she is horizontal, she does not emit semen.)

Objection: This implies that Rava discusses a woman who walked to the Mikveh. We should assume that this caused all the semen to leave!

Answer #2: Also Rava discusses a woman who was carried to the Mikveh on her bed;

Version #1: The Beraisa discusses a woman who did not sit up. Rava discusses a woman who sat up.

Version #2: The Beraisa discusses a woman who was still during Bi'ah. (All the semen is absorbed together, so it is all discharged together.) Rava discusses a woman who turned over during Bi'ah. (end of Version #2)

Question (Rav Shmuel bar Bisna): If semen exudes from a woman, is she Teme'ah k'Ro'eh, or only k'Noga'as?

If she is k'Ro'eh, it is Soser (any clean days that she counted). Any amount of semen is Metamei her, and she becomes Teme'ah before it leaves her;

If she is k'Noga'as, it is not Soser, she is Teme'ah only if a Shi'ur (the size of a lentil, or the opening of the Ever) exuded, and she is Tehorah until it leaves her.

Answer (Abaye, Rava, and Rav Yosef): She is k'Ro'eh.

Support (Abaye, for himself): R. Shimon says that her law is not more stringent than her husband only regarding Tum'as Beis ha'Starim. However, it is Soser, and any amount is Metamei her.

Shabbos 86a (Mishnah) Question: What is the source that if a woman emits semen (within) three days (after Bi'ah), she becomes Teme'ah?

Answer: "Heyu Nechonim li'Shloshes Yomim." (Bnei Yisrael abstained from relations three days before Matan Torah, lest women become Tamei due to emitting semen. After three days, semen putrefies and loses its Tum'ah.)

Rishonim

Rambam (Hilchos Isurei Bi'ah 6:16): If a woman emitted semen during her count, this is Soser one day. This is like a Zav who saw Keri. It is Soser one day.

Hagahos Maimoniyos: This is like Abaye, Rav Yosef and Rava. Rava wanted to prove that it is not Soser, but he was refuted. Also R. Chananel connotes that Rava accepted the refutation, and Rami's question was not resolved. Even if Rava did not accept it, we do not rule like him. Therefore, if a woman had Bi'ah and then saw blood or a Kesem (stain), she must wait six full Onos (half-days) after the Bi'ah, lest she emit semen and be Soser that day. Therefore, she may not begin to count until the fifth day. Towards the end of the fourth day, she checks and is Mafsik b'Taharah and begins counting at night. If she wants to start counting the day after she saw, she must clean the area very well with a wad or fine cloth to remove the semen. If she wants to avoid all Safek, she also bathes in hot water. We do not rely on walking to expel all the semen. We say that walking can expel all the semen, but it is not certain. All this is from Sefer ha'Terumah and Sefer ha'Mitzvos. The Ramban says that walking expels everything that there is no expel.

Rosh (4:1): Rava (who said that Rami bar Chama's question was improper) is rejected. He cannot answer why semen interrupts only one day of a Zav's count. Rami's question was not answered. Therefore, we are stringent about one who emits semen. She interrupts one day. Abaye, Rav Yosef and Rava all said that she is k'Ro'eh, even according to R. Shimon, and surely according to Rabanan. It seems that Rava retracted. Rava expounded (37a) that Tum'ah may not interrupt in the seven clean days. Abaye expounded that Tum'as Zivah may not interrupt. This connotes that Rava holds that no Tum'ah may interrupt. Perhaps Tum'as Leidah is like Tum'as Zivah, for it is Metamei for seven days, and causes Tum'as Mishkav u'Moshav, unlike an emission of semen. Perhaps Rava retracted about it. Therefore, no woman may begin counting the seven clean days less than five days from her last Bi'ah. E.g. if she had Bi'ah on Motza'ei Shabbos, she may not begin counting the seven clean days until Thursday. We hold like Rabanan (Shabbos 86a), who require six complete Onos for emissions. I.e. semen does not putrefy in less than this time. If she would emit semen on Tuesday night, earlier than the time she had Bi'ah on Motza'ei Shabbos, it is Soser. Therefore, she begins counting on Wednesday night.

Rosh: If she wants to start counting the day after she saw blood, she cleans herself very well with a wad or fine cloth to remove all the semen, or bathes in hot water, which expels all the semen. A Mishnah (Mikva'os 8:4) says that if she immersed and then cleaned herself, the Tevilah did not help (perhaps she emitted semen right afterwards). This implies that if she cleaned herself first, we are not concerned for emissions. A Bas Kohen widowed from a Yisrael immerses, and she may eat Terumah at night (Yevamos 69b). Rava forbids for three days! We must say that in Yevamos, she cleaned herself. Also Shabbos 86a and Nidah 54a connote that cleaning helps expel the semen. If a woman sees every alternative day, she may have Bi'ah on the eighth day and night, and four nights among every 18 days. During the days of Zivah, she sees, guards the next day, immerses at night and she is permitted. One may not be lenient to say that walking surely emits all the semen. Perhaps it does!

Rosh: The Ra'avad says that one who emits semen is Soser only for Taharos, but not for Heter Bi'ah. Regarding Koshi, we say (37a) that only something that causes (Tum'ah) is Soser. Since emitted semen does not forbid Bi'ah, it is not Soser for Bi'ah. This is astounding. How can it be Soser half-way, i.e. for Bi'ah she can immerse, but she is still Temei'ah regarding Taharos? Rashi explained that Keri does not cause a man to be a Zav, but it is Metamei for one day, therefore it is Soser one day. The Ra'avad brought another proof from 67b. Chachamim forbade to immerse on the last clean day and have Bi'ah, lest she see blood. (It is Soser retroactively.) They did not forbid lest she emit semen! This shows that semen is not Soser. Also, it is unreasonable that the Torah would permit only on condition that she not turn over or walk at all (after the Bi'ah, until the end of the day).

Rosh: This does not prove that semen is not Soser the last day. Surely, blood or Zov, which are Soser the entire count, are Soser until the end of day seven. An emission of semen is Soser only one day, so after she counted part of the last day, it is not Soser. Semen is Soser primarily because the day was not clean. Once she counted part of the last day and immersed, the clean days finished, so semen is not Soser. We find that Zov is Soser on the last day, but semen is not (Zavim 1:2). I found that R. Yonah says like I said. He proves this from Dam Nidah, which is not Soser according to Beis Shamai, who requires one who saw on day 11 of Zivah to guard a day. All the more so semen is not Soser! It is proper to be stringent like R. Yonah. This is the custom in France and Ashkenaz.

Poskim

Shulchan Aruch (YD 196:13): If a woman had Bi'ah, saw blood, and the blood ceased, and she wants to count seven clean days starting the day after she saw, she must clean the area very well with a wad or fine cloth to remove all the semen, or bathe in hot water, which expels all the semen.

Beis Yosef (DH u'Mah she'Chasav Emnam): The Rosh and Rashba say that if a woman cleans herself properly after Bi'ah, she may begin counting the next day.

Beis Yosef (DH u'Mah she'Chasav sheha'Ramban): The Ramban says that walking expels all the semen. A Mishnah teaches that if she immersed before cleaning herself, it did not help. The Rashba says that the Ramban must explain that this is when she did not walk. Some explain that "Kivdah Es ha'Bayis" is a euphemism for cleaning herself. Others explain that it literally means that she swept the house, for she bends and spreads her thighs and expels everything. Alternatively, she walked and all the semen went to Beis ha'Chitzon, and the walls of the womb kept it in. She must clean herself for Taharos due to touching semen, but we are not concerned for an emission of semen that is k'Ro'eh.

Beis Yosef (DH u'Mah she'Chasav v'Lo): The Rashba requires cleaning and bathing, to avoid all Safek.

Rema: Some say that we are not experts nowadays, and we do not rely on this. This is the custom. Our custom is to wait even if she did not have Bi'ah at all, in order not to distinguish between one count and another, and all the more so in such a case. Anyone who breaches a fence in matters in which the custom is to be stringent, a snake will bite him.

Beis Yosef (DH Aval): Semak says that we are not experts to rely on this. He does not rely on even cleaning and bathing. Hagahos Maimoniyos connotes like this. We do not know how far the water or cloth must reach. No (other) Posek says so, so we may rely on cleaning or bathing in hot water.

Shach (25): How does this depend on expertise?! It was a decree to always require six Onos even for one who did not have Bi'ah, due to one who did. Indeed, cleaning and bathing do not help. This is no better than a woman who did not have Bi'ah at all! Chachamim decreed about a common matter. Cleaning and bathing is relevant to the previous Sa'if, i.e. a woman who erred and immersed and had Bi'ah a day too early. She must wait six Onos before counting one more day (that she missed). Through cleaning and bathing, she need not wait. The Rema agrees to this. However, in practice, since Semak and Hagahos Maimoniyos say that we are not expert, l'Chatchilah one should not be lenient. Also the Bach says so.