Gemara

"Ki Yazov Zov Damah" includes Ones.

41a (Mishnah): A woman becomes Teme'ah when blood (leaves the Makor and) enters the Beis ha'Chitzon.

Question: What is the Beis ha'Chitzon?

Answer #1 (Reish Lakish): It is the place visible when a girl sits. (The Rechem opens a bit.)

Objection (R. Yochanan): That is considered exposed! (It is not "bi'Vesarah".)

Answer #2 (R. Yochanan): It is the place between the "teeth" (protrusions where the Rechem meets the Prozdor);

(Beraisa - R. Zakai): She is not Tamei until blood passes between the teeth. Between the teeth is like inside the Rechem.

Answer #3 (Beraisa): It is the place of Dishah.

(Rav Yehudah): This is where the Ever rubs against the walls of the Prozdor.

(Beraisa): "Bi'Vesarah" teaches that she is Teme'ah even though the blood is still inside her.

57b (Shmuel): "Bi'Vsarah" teaches that she is Teme'ah only if she felt it in her body.

Avodah Zarah 18a: It was decreed that R. Chanina ben Tradyon's daughter sit in a harlot's booth. R. Meir dressed up like a soldier, and requested her services.

R. Chanina's daughter: I am Nidah!

R. Meir: I will wait for you!

R. Chanina's daughter: Why wait? There are girls here prettier than me!

Rishonim

Rambam (Hilchos Isurei Bi'ah 9:1): A woman is Temei'ah mid'Oraisa only if she was Margish and saw that blood left her. If she was not Margish and checked and found blood inside the Prozdor, the Chazakah is that it came with Hargashah.

Question: Does the Isur Nidah apply only to one's wife, or even to a Penuyah (single woman)? The Rashba and Ra'avad did not discuss this. People say that a Penuyah is permitted. Why did Chachamim allow harlots? Presumably, she does not Metaher herself. Why didn't they make an enactment for Penuyos, lest many stumble with her? There is Kares for Bi'ah with her, and one who touches even her finger is Chayav lashes! The Rambam connotes that the Isur Nidah is even for a Penuyah.

Answer (Rivash 425): Clearly, it applies even to a Penuyah. One who rapes or entices a Nidah (Penuyah) pays a fine, even though there is Kares (Kesuvos 29a). The verse forbids (pleasurable) closeness with (any) Nidah woman. Chachamim decreed fences to distance one from his wife when she is Nidah. They did not need to decree about a Penuyah, for she is always forbidden. The Rambam forbids mid'Oraisa, if she prepared herself for this. All forbid seclusion, and all the more so Bi'ah, at least mid'Rabanan! Chachamim did not permit harlots. It seems that the harlots used to be Metaher themselves, like we find regarding R. Chanina ben Tradyon's daughter.

Note: I do not understand his proof. Rashi connotes that she claimed to be menstruating. (Bi'ah would not be pleasurable amidst the blood.) Surely, R. Meir did not suggest waiting up to a week until she immerses! Further, how can we learn from a Tzadekes forced to sit in a harlot's booth, to immoral women who desired to be harlots?!

Rivash: Chachamim did not enact that Penuyos immerse, lest the Isur be light in people's eyes. In the days when there were Taharos, they would immerse minors.

Terumas ha'Deshen (Kesavim 108): The Semag says that we do not wear Tefilin on Shabbos, for we already have two Osos (signs that we are Hash-m's nation, i.e. Bris Milah and Shabbos). If so, if one did not circumcise because his brothers died due to Milah, he should wear Tefilin on Shabbos! Rather, Semag merely explained an Agada. We do not derive laws from it. Chazal said (Nidah 31b) that Milah is on day eight, lest everyone else be happy and the parents be sad (for she still has Tum'as Yoledes). Will we say that if she was Yoledes b'Zov and is forbidden until seven clean days (or a twin sister was born with the boy), we delay the Bris until day 14?! There are many matters like this in the Gemara.

Poskim

Shulchan Aruch (YD 183:1): If blood left a woman's Makor, whether b'Ones or willingly, she is Temei'ah if she felt it leave. However, once she feels that it left her Makor, she is Temei'ah even though it did not leave her body.

Beis Yosef (DH u'Mah she'Chasav Bein): She becomes Temei'ah even due to Ones, e.g. she jumped from place to place, or she saw animals engaging in relations and she had desire and saw blood, and all similar cases.

Taz (1): 'Willingly' means like her normal nature, due to herself.

Shach (2): Nidah mid'Oraisa is only if she had a Hargashah. Mid'Rabanan she is Temei'ah even without Hargashah.

R. Akiva Eiger: The Shach connotes that blood without Hargashah is not Soser (does not interrupt the count of seven clean days). See the Shach (196:13, citing Darchei Moshe, who discusses whether a Kesem less than a Gris is Soser) and (Sidrei Taharah 194:4).

Shulchan Aruch (ibid.): Even if she saw only a drop like a mustard seed, she sits (waits for) seven clean days for it.

Shach (3): The same applies to blood less than a mustard seed.

Gra (3,5): In this Sa'if, the Shulchan Aruch gives the law mid'Oraisa. However, to sit seven clean days is mid'Rabanan.

Pischei Teshuvah (1): There are three kinds of Hargashos that make a woman Temei'ah mid'Oraisa. 1) Her entire body is shaken, like the Rambam says (Hilchos Isurei Bi'ah 5:14). 2) Her Makor opened, like it says in Siman 188 and 190:1. 3) Teshuvos of Acharonim discuss feeling a liquid flowing internally (Noda bi'Yehudah YD 55, Chavas Da'as 190:1). However, the Chasam Sofer (145, 153) says that this is not a Hargashah, and he rules like this in practice, and so he received from R. Nasan Adler. If she found blood in a Bedikah without Hargashah, or after Bi'ah without Hargashah, it is a Torah Safek. Perhaps she was not Margish because she thought that she felt the cloth or Ever. Shev Yakov, Keser Kehunah and the Shlah say so. However, the Kreisi u'Pleisi (1), Sidrei Taharah and Chavas Da'as say that it is mid'Oraisa. However, if she merely wiped herself or did not enter the cloth deeply, only from the Prozdor and outside, it is only mid'Rabanan if there was no Hargashah. The same applies to blood found after urinating (Chavas Da'as).

Pischei Teshuvah (2) Me'il Tzedakah (34) was unsure if the primary Tum'ah is when the blood goes from the Makor to Beis ha'Chitzon, but leaving the body does not matter, and she can begin counting the seven clean days once it entered Beis ha'Chitzon (even before it left the body). Or, also leaving the body is considered seeing blood, so it cannot count like a clean day. Bris Avraham (YD 42) proved that leaving the body is not significant, but the question does not depend on this, for she needs a Hefsek Taharah, so the Beis ha'Chitzon must be clean of all blood. If we would know that no more blood entered Beis ha'Chitzon, what was there already would not be Metamei her. However, unless she checks the entire Beis ha'Chitzon, we are concerned lest more blood came from the Makor, since it is established to be open (flowing). However, this affects blood that left the Makor unlike a sighting of blood. We are not concerned for when it leaves the body.

Yad Avraham (1): Tif'eres Yisrael (on Kreisi u'Pleisi, 1) concluded like the Rambam (above) and Terumas ha'Deshen (2:47), that blood on a Bedikah cloth without Hargashah is a Torah Safek, for she attributes the Hargashah to the cloth.

Rema: There is no distinction between a Penuyah or a married woman regarding the Isur. Anyone who has Bi'ah with a Nidah is Chayav Kares.

Gra (6): Also Kerisus 14a teaches this.

Note: It says that one is liable multiple Chata'os for Bi'ah with... another man's wife who is a Nidah. Since the Isur Nidah takes effect on one who is already forbidden due to Eshes Ish, all the more so it takes effect on a Penuyah!

Kreisi u'Pleisi (3): R. Meir (31b) expounded that a Nidah is forbidden for seven days, lest a man be too regular with his wife, and she will become repulsive to him. After being forbidden for seven days, she will be as dear to him as when he first married her. Minchas Yakov (4, on Toras ha'Shelamim, and Siman 192) derived that the Torah does not obligate a Penuyah to wait seven days. If so (the verse does not apply to her), there is no Kares! This is a total mistake. Even a Shifchah, who cannot marry, is Tamei for seven days. Perhaps one might have thought that the Torah should be more lenient for a married woman, to enable Peru u'Rvu. R. Meir teaches why this is not so!

Shiyurei Berachah (on Birkei Yosef, 1): Minchas Yakov and Kreisi u'Pleisi overlooked the Terumas ha'Deshen (who says that we do not derive Halachos from Agados). The Yerushalmi says that we do not learn from Medrash. Kal va'Chomer we do not learn based on reasons, especially for an Isur Kares.