Gemara

Question: If a girl did Mi'un, she could not give birth (from her first husband, since she was a minor)!

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

A minor does so, lest she get pregnant and die. A pregnant woman does so lest the fetus get mashed. A nursing woman does so lest pregnancy force her to wean her baby, and it will die.

R. Meir says, a girl between 11 and 12 years uses a wad. If she is younger or older, she has relations normally.

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

Nidah 3a: Shamai holds that a woman who sees Dam Nidah is not Teme'ah retroactively because had the blood come earlier, it would have left her body right away.

Question: If a woman has Bi'ah with a wad, perhaps it held in the blood!

Answer #1 (Abaye): Shamai admits that such a woman is retroactively Teme'ah.

Answer #2 (Rava): A wad shrivels up. It does not block the entire width of the womb, so it does not hold in blood.

Rishonim

The Rif and Rosh (Yevamos 2b and 1:7) bring Rav Bivi's Beraisa verbatim.

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

Rashi (Kesuvos 39a DH Meshamshos): The three women may use a wad at the time of Bi'ah. It is not as if they waste seed.

Nimukei Yosef (Yevamos 2b DH Meshamshos): Rashi holds that only the three women may use a wad at the time of Bi'ah. Others ask, if so why is it forbidden before 11 years? Since in any case she cannot become pregnant, the wad does not waste the semen! Therefore, they say that the three women must use a wad due to danger to her or the fetus.

Tosfos (12b DH Shalosh): Rashi says that only these three are permitted. 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. The Ri supports Rashi from Nidah (13a), which praises a woman who checks herself (for blood) 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, this refers to causing it to leave the body, but not after it left

Avnei Nezer (EH 81:4): The three women normally do not become pregnant. They are not forbidden to have Bi'ah without a wad. Rashi holds that if we do not permit with a wad, they will have Bi'ah without a wad, therefore we permit a wad. However, if pregnancy would be dangerous for a woman who is prone to become pregnant, she may not have Bi'ah without a wad. Therefore we need not permit her and she will not have Bi'ah.

Rashi (Nidah 3a DH Meshameshes): The Gemara asks about one of the three women who are permitted to use a wad (why she is Teme'ah retroactively).

Rebuttal (Ramban Nidah 3a DH Meshameshes): All three are among the four women who are never Teme'os retroactively (Nidah 2a), for they do not normally see blood! Rather, we discuss a regular woman who used a wad.

Defense #1 (Beis She'arim 50 DH v'Yesh Lehavi): Retroactive Tum'ah does not apply to their first sighting, but if they already saw once, it applies to future sightings (Nidah 7b). It is difficult to say that we ask about a woman who used a wad b'Isur.

Defense #2 (Igros Moshe EH 1:63 DH v'Lachen): Even Chachamim permit a wad in a case where there is a special danger.

Achi'ezer (1:EH 23 DH v'Hinei): The Halachah follows Chachamim, not R. Meir who permits the three! Rashi must hold that R. Meir permits the three, and therefore obligates them. Chachamim permit but do not obligate them.

Yam Shel Shlomo (1:8 DH Piska): Even though the Mordechai and Ran hold like R. Tam, it seems that Rashi is correct. The Gemara in Nidah discusses a wad left in all the time. Bi'ah with a wad is not like spilling one's seed onto rocks. It is normal Bi'ah; she enjoys it. However, it also seems that R. Tam is correct that the three must use a wad, and others may. Women may waste seed, since they are not commanded about Peru u'Rvu. Perush ha'Rosh says that the three women must put a wad; this connotes they put it there before Bi'ah.

Hagahos Ashri (Kesuvos 3:4): The Ri permits only a minor or a woman who is pregnant or a nursing to use a wad, but not others. Chachamim do not permit even these, whether the wad is inserted before or after Bi'ah. R. Tam permits.

Rashba (Teshuvah 1199 DH v'Od Ani): It is very rare for a minor to give birth, therefore they permitted her to use a wad.

Poskim

Shulchan Aruch (EH 23:5): If a woman has an obstruction in her womb that causes her husband to spill his semen outside, Bi'ah is forbidden.

Source (Beis Yosef DH Kosav): The Rosh (33:3) forbids this because he always spills his seed. This is "V'Shiches Artzah" (Bereishis 38:9). This is worse than a wad. With a wad is normal Bi'ah, even though it cannot impregnate, just like Bi'ah with an old woman or minor.

Pischei Teshuvah (2): Doctors said that pregnancy would be a mortal danger for a certain woman. Chemdas Shlomo (46) permitted her to have Bi'ah with a wad at the time. R. Akiva Eiger and the Chasam Sofer permitted a wad only afterwards.

Igros Moshe (EH 1:63 DH v'Od): Birth is always a danger for which we Mechalel Shabbos. It is a Mitzvah to have children; we rely on a miracle because it is the way of the world. This is why Chachamim rely on "Shomer Pesa'im Hash-m" also for the three women. It is unreasonable that other women must rely on "Shomer Pesa'im", but the three women are forbidden to rely on it. The Ro'oh and Ritva say that we do not infer that the three must rely on "Shomer Pesa'im" and may not use a wad. However, if there is a special danger for a particular woman, Chacham permit and obligate her to use a wad. The way the Ri explains Rashi, R. Meir permits fulfilling Mitzvas Onah with a wad if it is needed. This is not the Isur of spilling seed. Even if a woman gets Hana'ah through Bi'ah Lo k'Darkah, there is pain and a man is not Yotzei Onah this way. Chachamim forbid only because they say that pregnancy is not categorically more dangerous than for other women. Surely those who permit Bi'ah Lo k'Darkah with semen would permit Bi'ah with a wad when there is a special danger. Perhaps Bi'ah with a wad is more lenient, and all would permit in this case.

R. Akiva Eiger (Teshuvah 1:71): I do not permit a woman for whom pregnancy is dangerous to insert a wad before Bi'ah. No one explicitly permits this. The Nimukei Yosef challenged Rashi because she is not forbidden to waste seed. He did not say that it is forbidden for her husband to have Bi'ah with a wad at the time.

Chut ha'Meshulash (3:8 DH v'Hen): According to Rashi, if a wad is permitted only for the three women due to the danger, they should be obligated to do so! We must say that it is not common for them to get pregnant; we are not concerned for the minority.

Igros Moshe (EH 1:3 DH Ha): Rashi (Yevamos 42b DH Akarah) defines 'Akarah' to be one whose womb was removed. The Gemara permits her to remarry (after Havchanah). Perhaps this is because externally the Bi'ah looks normal, just like Rashi permits a wad at the time of Bi'ah for the three women, but R. Tam would forbid. However, no one explicitly disagrees with Rashi about one without a womb, so we may permit it. Perhaps even R. Tam forbids only when a foreign substance is used to absorb the semen.

Igros Moshe (EH 1:13 DH Aval): When pregnancy is dangerous, Acharonim permit Bi'ah while a rubber is in place. This is only if she is modest and will not publicize this, lest women for whom there is no danger do the same.

She'alas Ya'avetz (1:14 DH v'Im): We do not marry off a girl before adulthood (Kesuvos 57b). This is even if she has Da'as, lest she get pregnant and die. The Halachah follows Chachamim who do not permit using a wad. B'Di'eved, if she already married we rely on "Shomer Pesa'im Hash-m", but not l'Chatchilah.