Gemara

3a (Beraisa - R. Yishmael): "He (a Kohen) will become Tamei for her (his deceased sister)" is optional;

R. Akiva says, it is an obligation.

23b (Mishnah): What differences are there between a (male) Kohen and a Bas Kohen? A Bas Kohen may become Tamei Mes. A Kohen may not...

23b - Question: What is the source of this?

Answer: It says "Benei Aharon", not Benos Aharon.

Yevamos 29b (Beraisa): If an Arusah (a wife before Nisu'in) died, her husband is not an Onen (forbidden to Kodshim) due to her. He does not become Tamei for her. If he died, she is not an Onenes, and does not become Tamei for him.

Kidushin 35b (Abaye): A verse forbids Kohanim to shave, and another forbids Yisre'elim to shave. We learn a Gezeirah Shavah "Pe'as-Pe'as" between them. Just like the former does not apply to women (we learn from "Benei Aharon"), also the latter does not apply to women.

Question: It says "Benei Aharon" regarding Tum'ah. If it also applied to shaving (to exclude a Kohenes), there would be no need for the Gezeirah Shavah! The Torah gave extra Mitzvos to Kohanim, yet female Kohanim are not forbidden to shave. All the more so, the Isur for Yisraelim to shave does not apply to women!

Answer: We would have said that "Benei Aharon" does not apply to shaving, for there is an interruption (a verse in between that applies even to women). We know that this is not so only due to the Gezeirah Shavah. (If female Kohanim were forbidden to shave, the Gezeirah Shavah would not teach anything!)

(Isi - Beraisa): Women are also exempt from Karachah (tearing out hair due to a Mes).

36a (Abaye): Isi learns a Gezeirah Shavah "Karachah-Karachah" from the Isur Karachah for Kohanim. Just like that does not apply to women (it says "Benei Aharon"), also the Isur Karachah of Yisraelim does not apply to women.

Question: If "Benei Aharon" exempts Kohanos from Karachah, we do not need a Gezeirah Shavah! Yisre'elim do not have extra Mitzvos, so all the more so, the Isur Karachah for Yisraelim does not apply to women!

Answer: We would have thought that "Benei Aharon" does not apply to Karachah, for there is an interruption. The Gezeirah Shavah teaches that this is not so

Zevachim 100a (Beraisa): The wife of Yosef ha'Kohen died on Erev Pesach, and he did not want to become Tamei. The other Kohanim forced him to become Tamei against his will.

Rishonim

Rambam (Hilchos Evel 2:6): Aveilus is a very severe Mitzvah. It overrides Tum'as Kohen for his relatives, so he can deal with them and mourn over them - "Lah Yitama." This is a Mitzvas Aseh. If he does not want to, we force him. This refers to a male Kohen. A Kohenes is not forbidden to become Temei'ah, therefore she is not commanded to become Temei'ah (for relatives). It is optional.

Rebuttal #1 (Ra'avad): An Arusah need not become Temei'ah for her husband. This implies that a Nesu'ah must become Temei'ah for him.

Rebuttal #2 (Ramach): A Kohen is forbidden to become Tamei, yet he is commanded to be Mitamei for relatives. All the more so, a Bas Kohen, who may become Tamei, is commanded to be Mitamei for relatives!

Defense (Radvaz): We learn from "Benei Aharon" that a Kohenes becomes Tamei through Mesim, unlike a Kohen (Sotah 23b). It does not say that a Kohen is warned about Tum'ah, but a Bas Kohen is not! This is a clear proof for the Rambam.

Note: What is the proof?! Migdal Oz cites the Gemara (23b) to say 'a man is Mitamei for Mesim, but a woman is not.' Perhaps the Radvaz had this text, and our version of the Radvaz was 'corrected' to conform to our text.

Toras ha'Adam: Perhaps the Rambam expounds that "Yetamei" applies only to one for whom "Lo Yitama" applies. Alternatively, "Benei Aharon" excludes Benos Aharon from the entire Parshah.

Turei Even (Rosh Hashanah 16a DH Chayav): Abaye obligates a man to feed Shalmei Simchah to his wife. If she is Temei'ah, he is exempt. Surely she is not commanded to enable him to be Mesame'ach her!

Question (ha'Makneh Kidushin 35b DH b'Rashi Hifsik): Rashi says that the interruption (from Benei Aharon) is the Mitzvah of Tum'as Kerovim (it applies even to women. This is like R. Akiva.) The Rambam must say that Karachah interrupts. However, Isi exempts women from Karachah, and if not for a Gezeirah Shavah, he would have said that there is an interruption!

Answer (ha'Makneh): The Rambam holds that if not for a Gezeirah Shavah, we would have obligated woman to be Mitamei, for the Mitzvah applies at all times. Rather, the Gezeirah Shavah exempts women from shaving. Now, it seems that the entire Parshah is only for Benei Aharon, and women are exempt also from Tum'as Kerovim. Chachamim obligate them regarding Karachah due to a Drashah (Am Kadosh Atah).

Maharatz Chayos (23a): The Mishnah lists the differences between Kohanim and Kohanos. According to the Rambam, why was Tum'as Kerovim omitted? Sefer ha'Mitzvos (37) says that Tum'as Kerovim depend on Kedushas Kohen. Therefore, it suffices to teach that she may be Mitamei to Mesim.

Torah Temimah (Vayikra 21:1 (4)): "Lo Yitama... Ki Im li'Sh'ero..." shows that the Mitzvah of Tum'as Kerovim is part of the Isur to be Mitamei, i.e. to be Mitamei only for these relatives. This is why Sefer ha'Mitzvos counts them like one. One opinion in Kidushin 36a says that "Benei Aharon" excludes Benos Aharon from the entire Parshah. All the more so it excludes them from the entire Mitzvah of Tum'ah!

Rosh (Hilchos Tum'ah 4): Rashi (Kesuvos 53a DH v'Lo Mitam'es) explains that since a Kohenes is not commanded about Tum'ah, 'an Arusah is not Mitamei' means that she need not be Mitamei. It applies also to a Yisraelis. It is a Mitzvah to be Metamei for relatives, but not for an Arus. The Rambam says that since a Kohenes is not commanded about Tum'ah, she need not be Mitamei for relatives. It is optional. Even a Nesu'ah is not Mitamei. The Beraisa discusses an Arusah due to other laws (which apply only to Eirusin). The Ramban strove to find a source for the Rambam, and failed. One must bring a proof to exempt women from this Mitzvah.

Question (Minchas Chinuch 264 [23]/ p.76 DH v'Hinei): The Rambam holds that a Kohen is Metamei for his wife only because it is like a Mes Mitzvah. In any case, there is no reason for a Nesu'ah to be Mitamei for her husband!

Poskim

Shulchan Aruch (YD 373:3): All relatives for whom a Kohen may become Tamei, it is a Mitzvah to be Mitamei for them. If he does not want to, we force him. The same applies to men and women.