Gemara

13a (Mishnah): The more a woman checks herself (for Dam Nidah), the better;

This is because she is not Bas Hargashah.

41b (Beraisa): "Yihyeh" teaches that if a woman discharges (a man's) semen (that had entered her Makor, and it goes to the Beis ha'Chitzon), she is Temei'ah;

R. Shimon says, semen is not Metamei her more than it is Metamei a man, i.e. until it leaves his or her body;

Semen during Bi'ah is Metamei her, even though it is Tum'as Beis ha'Starim.

Question: Even without a discharge, she is Teme'ah due to Bi'ah!

Answer: If she immersed after Bi'ah, she becomes Tehorah at night. The discharge of semen is Metamei her again.

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

Answer #1: R. Shimon discusses a woman who was carried to the Mikveh on her bed. (She was horizontal the entire time, so we are not concerned for a discharge of semen.)

Objection: This implies that Rava discusses a woman who walked to the Mikveh. Perhaps this caused all the semen to leave. Why does Rava say that surely she will discharge semen?

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

R. Shimon discusses a woman who did not sit up. Rava discusses a woman who sat up.

45a (Rav Bivi - Beraisa): Three women use a contraceptive wad to (absorb the semen and) prevent pregnancy (for it is dangerous for them): a minor, a pregnant woman, and a nursing woman.

Chachamim say, at any age she has relations normally. Shamayim will have mercy on her - "Hash-m guards the simple."

Rishonim

Perush ha'Rosh (Nedarim 35b DH Shalosh): The three women must put a wad there to absorb the semen to prevent pregnancy.

Tosfos (13a DH Nashim): R. Tam explains that the three women (for whom pregnancy is dangerous) must use a contraceptive wad at the time of Bi'ah. Other women may do so, for they are not commanded about Peru u'Rvu. We can explain that women have no Isur to waste seed, even if they would emit seed due to sensation. However, even though they are not commanded about Peru u'Rvu, they are equated to men regarding all punishments (so wasting seed should apply to them)! Rather, the Isur does not apply to women because they are Mazri'a internally (their seed remains in the bodies), whereas men are Mazri'a externally.

Tosfos (Yevamos 12b DH Shalosh): Rashi permits only these three women to use a wad. Other women are forbidden, for it wastes the semen, even though women are not commanded about Peru u'Rvu. R. Tam says that a wad before Bi'ah is surely forbidden. Such a Bi'ah is like spilling one's seed onto rocks. A wad after Bi'ah is permitted. The Bi'ah is normal, like Bi'ah with a minor or Ailonis, who cannot become pregnant. She may use the wad afterwards, because she is not commanded about Peru u'Rvu. The three women must use a wad. The Ri supports Rashi from Nidah (13a), which praises one who checks herself often, for she is not 'Bas Hargashah'. I.e. this will not cause her to waste seed. This implies that she may not waste seed. Therefore, other women may not have Bi'ah with a wad. However, we could explain 'she is not Bas Isur Hargashah', i.e. she may waste seed.

Tosfos ha'Rosh (Yevamos 12b DH Shalosh): Wasting semen is like bringing a flood to the world. It is unreasonable to permit using a wad during Bi'ah for Simchas Onah. It is better than she refrain from Bi'ah! After Bi'ah any woman may use it. Even if she is commanded about wasting seed, the Isur for a man or woman is causing seed to leave one's body. There is no Isur after seed left the body (in which it originated).

Tosfos (Sanhedrin 59b DH v'Ha): What is the source that Nochrim are not commanded about Peru u'Rvu? Even though it is not counted among their seven Mitzvos, we said (58b) that only Lavim are included in the seven! Really, also Peru u'Rvu is a Lav, for whoever is commanded about Peru u'Rvu is commanded not to waste seed.

Rashba (13a DH Kol): Tosfos says that whoever is commanded about Peru u'Rvu is commanded not to waste seed. This is like his opinion in Nidah, that wasting seed does not apply to women. The Ramban asked that even though they are not commanded Peru u'Rvu, they are forbidden to waste seed. Everyone was punished for this in the flood. Even though they have no Isur to castrate, they may not waste seed.

Mishneh l'Melech (Hilchos Melachim 10:7): Perhaps the Ramban holds that Nochrim are commanded Peru u'Rvu not from the verse said to Adam and Chavah, which was merely a Berachah, like the Mizrachi says. Rather, it is due to the verse said to Noach and his sons (Bereishis 9:1). They were commanded only after the flood! However, the Mishnah in Yevamos (65b) proves that Adam was commanded. Rather, the Ramban means that women were punished even though they are not commanded about Peru u'Rvu. Alternatively, he asks that if women may waste seed because they are not commanded Peru u'Rvu, a Nochri who fulfilled Peru u'Rvu should be permitted to waste seed. Surely Nochrim fulfill Peru u'Rvu after a certain amount of children, just like Yisraelim do. Rashi (Bereishis 4:24, citing a Medrash) says that Lemech's wives separated from him after they fulfilled Peru u'Rvu. However, what is the source that Nochrim are commanded about wasting seed? Also, why isn't it listed among their seven Mitzvos? This requires investigation.

Aruch l'Ner (13a DH Ke'ilu): The Ramban, Rashba and Ran prove from our Gemara that a woman is commanded about wasting seed, unlike R. Tam, for also women were punished in the flood. However, no place gives the warning not to do so. R. Tam can explain that it is included in Peru u'Rvu (which applies only to men). However, how can the opposing opinion answer? Even more difficult is what is the source that Yisre'elim are commanded about wasting seed? The Gemara expounds from Lo Sin'af, but this is a mere Asmachta. (The verse forbids adultery.) Perhaps we learn from Bava Kama (91b), which forbids wounding oneself due to 'Bal Tashchis.' One transgresses for destroying clothing, and all the more so for harming himself. The Gemara rejected this, for the body heals. It then learned from "Asher Chata (Al ha'Nefesh)." If so, we can say that 'Bal Tashchis applies to wasting seed. The generation of the flood was punished because Hash-m created the world to be settled, and they did not want to help do so.

Poskim

Shulchan Aruch (YD 335:10): A woman may attend to a man with stomach illness.

Birkei Yosef (5): Even though she will come to Hirhur if she serves him, we are not concerned, for she is not commanded about wasting seed, since she is not commanded Peru u'Rvu, like Tosfos says. Even if she wastes her seed, there is no problem. This is why a woman may wrap a dead man (352:3), but not vice-versa.

Magen Avraham (OC 606): (We immerse on Erev Yom Kipur.) If a woman had Bi'ah within three days before Yom Kipur she should wash the area with hot water to prevent an emission of semen. If the Bi'ah was close to the time of her Tevilah or Veses (when she normally becomes Nidah), which are the most likely times to become pregnant, she should not wash, lest this prevent pregnancy.

R. Akiva Eiger (Teshuvah 1:71): Seemingly, a wad after Bi'ah should be permitted. Rashi and Tosfos do not argue about the Halachah, only about how to explain the Gemara. The Ri suggested a source for Rashi to forbid other women to waste seed. We do not abandon what was clear to R. Tam due to the Ri's Safek. However, most Poskim forbid. Rashi in Kesuvos connotes that adult women may not use a wad because they waste seed. This applies also after Bi'ah. The Nimukei Yosef asked that wasting seed does not apply when she cannot become pregnant. The Shitah Mekubetzes in Kesuvos brings many Rishonim who hold like the Nimukei Yosef who permit a minor to use a wad for in any case she cannot become pregnant, but forbid an adult for it wastes the seed.

R. Akiva Eiger (ibid., DH v'Hayah): Kesuvos (71b) connotes that a woman is not forbidden to waste seed. Rav Yehudah explains the Mishnah to discuss one who annulled his wife's vow on condition that she remove the semen after Bi'ah to prevent pregnancy. If this were forbidden, according to Abaye his stipulation would be Batel, for it is contrary to Torah! However, Abaye could explain the Mishnah differently, like the Beraisa there.

R. Akiva Eiger (ibid., DH v'Nir'eh): Nidah 42a shows that a woman may not waste seed. Rava is Metamei a woman for three days after Bi'ah, for she will surely emit semen. He explains that the verse discusses a woman who never sat up, therefore we are not concerned for emissions. If it were permitted for a woman to remove the semen, Rava would not establish the verse in such a bizarre case (she was carried to the Mikveh on the bed)! The Magen Avraham is against the majority of Poskim.

Suggestion: Bedikos show that women may use a wad after Bi'ah! (Chachamim enacted that Benos Yisrael check themselves for Dam Nidah every morning and evening for the sake of Taharos, even though this removes the semen - Nidah 4b!)

Rebuttal (R. Akiva Eiger 72): Surely R. Yochanan ben Brokah agrees that Bedikos were encouraged, even though he obligates women in Peru u'Rvu (Yevamos 65b)! A Bedikah does not necessarily prevent pregnancy. Perhaps the semen was already absorbed. He must hold like R. Shimon, who permits Davar she'Eino Miskaven. However, even R. Yehudah who forbids Davar she'Eino Miskaven permits Bedikos. This shows that women may waste seed.

Chavos Ya'ir (31 DH v'Al): R. Tam holds that after semen has been absorbed, she may not ruin it. Yevamos 100b supports R. Tam. It brings Rav Bivi's Beraisa to show that a Mema'enes will not give birth (i.e. because a minor must use a wad). However, one could say that since it is permitted, surely all minors do so to avoid the danger. Rashi forbids other women to use a wad even after Bi'ah, for the man knows that his seed will be wasted. Even though a woman is exempt from Peru u'Rvu and may drink a sterility potion, perhaps she may not overtly waste seed, due to Lasheves Yetzarah (a Mitzvah mi'Divrei Kabalah to have children). One must be concerned for the stringent opinions.