Gemara

Rav Kahana - Beraisa #1: Repositories for straw or wood, a Beis ha'Bakar (a cattle pen) or storehouses are exempt from Mezuzah because women Na'osos, i.e. bathe there.

Objection (Rav Yehudah): This implies that Stam (if women do not bathe there), they are obligated - but a Beraisa exempts!

Beraisa #2: A Refes (stall) is exempt from a Mezuzah (even though women do not bathe in a Refes, for cattle are constantly there and it is filthy from excrement - Tosfos DH veha'Tanya Refes).

Answer #2 (Rav Yehudah): 'Na'osos' means that women perfume themselves there. Beraisa #1 exempts storehouses... even though women perfume themselves there.

Beraisa #3: "Beisecha" - only places specifically for you (to dwell in) require Mezuzos, but not a repository for straw or wood, Beis ha'Bakar or storehouse;

Some obligate them.

A tradition from Sinai exempts a Beis ha'Kisei, tannery, bathhouse, Mikveh or a place where women Na'osos.

Rav Kahana explains, these Tana'im argue about Beis ha'Bakar and storehouses Stam; the tradition exempts a Beis ha'Kisei,...or a place where women bathe.

Question: Why must the Beraisa exempt a bathhouse and a place where women bathe? These are the same!

Answer: One might have thought that only public bathhouses are exempt, for they are very filthy, but private bathhouses are obligated, for they are not so filthy. The Beraisa teaches that this is not so.

Rav Yehudah explains, the first Tana exempts Beis ha'Bakar and storehouses even when women perfume themselves there; the latter Tana obligates in this case. The tradition exempts filthy places even if women perfume themselves there.

11b - Beraisa - Suggestion: Perhaps a Beis ha'Kisei, tannery, bathhouse, or Mikveh requires a Mezuzah!

Rejection: "Beisecha" - only places for dignified residence are obligated.

Rishonim

Rif and Rosh (Hilchos Mezuzah 6a and 15, respectively; these are found after Menachos): A Beraisa exempts a Beis ha'Kisei, tannery, bathhouse, and Mikveh from "Beisecha". A storehouse or Beis ha'Bakar where women bathe is exempt.

Rambam (Hilchos Mezuzah 6:9): The following places are exempt from Mezuzah because they are not for dignified residence - a Beis ha'Kisei, tannery, bathhouse, Mikveh, or similar places.

Poskim

Shulchan Aruch (YD 286:1): Mezuzos are required for a Refes Bakar or coop.

Question: A Refes is filthy, it should be exempt like a tannery or bathhouse!

Answer #1 (Tur): Any place of residence is obligated even if it is filthy. It is good to cover the Mezuzah so it will not be seen.

Answer #2 (Shach 2): A Refes is dirty, but it is not so filthy.

Aruch ha'Shulchan (6): In the days of the Gemara the Refes was clean and excrement was not kept there a long time. Nowadays in most places they are filthy, and sometimes people use it for a privy, so it is just like Beis ha'Kisei. If there is much filth in a coop it is exempt.

SMaG (cited in Beis Yosef DH u'Mah she'Chosav Aval): Some cite the Yerushalmi to exempt a city in which there are pigs.

Rebuttal (Beis Yosef, ibid.): We do not find such a Yerushalmi. Even if there is such a Medrash, doorways where pigs are not frequent would be obligated. Even where pigs are frequent, one may place a Mezuzah if the bottom of the top third of the doorway (the proper place for a Mezuzah) is at least 10 Tefachim above the ground. There is a great punishment for failing to have a Mezuzah.

Shach (2): If people pass through a place without filth, e.g. a storehouse, this is considered Dirah and it is obligated.

Shulchan Aruch (ibid. 2) Repositories for straw or wood and a Beis ha'Bakar are obligated unless women bathe there. It is disrespectful to have a Mezuzah where they are naked.

Aruch ha'Shulchan (10): If they bathe there only occasionally this does not exempt from a Mezuzah. However, at these times one must put an opaque cover on the Mezuzah. If one cannot avoid disgraceful activities near the doorway, the Mezuzah should be put on the inner part of the Mashkof (the upper protrusion which prevents the door from swinging out; when the door is closed it is covered), for the Tur and Rema say that b'Di'eved one is Yotzei there.

Rema: Only these are exempt, but a proper room is obligated even if a couple sleeps and has relations there. Some are lenient to exempt even a room used for relations.

R. Peretz (brought in Beis Yosef DH v'Chosav Mordechai): Our Gemara teaches that places where women launder, bathe and perfume themselves are exempt, and all the more so a room used for relations.

Rebuttal (Beis Yosef, ibid.): We exempt storehouses and pens when women bathe there, but we do not exempt a proper Dirah! Also, the exemption is only because women are naked there - this does not apply to where people have relations. People conduct like this (to have Mezuzos in proper rooms).

Defense (Bach DH Chosav): R. Peretz exempts because they bathe there even though they do needs of Dirah there, i.e. laundering and perfuming. Alternatively, they launder diapers, which is a disgrace to a Mezuzah. The Gemara exempted storehouses and pens where women bathe, for this was the norm. Surely, the same applies to a proper room in which they bathe. Nowadays, we only have Mezuzos only on the door to Reshus ha'Rabim because women bathe in different rooms of the house in winter and summer. Recently people started to put Mezuzos in all rooms. One must be careful not to bathe or launder diapers or have relations there. Even though they are covered at the time of relations, this is more stringent than being naked; a Mechitzah of 10 Tefachim in front of Kisvei ha'Kodesh does not permit relations, but one may be naked if such a Mechitzah is between him and Kisvei ha'Kodesh. People in the Beis Yosef's dominion conduct like he says - but Ashkenazim have Mezuzos only on the door to Reshus ha'Rabim. However, if when the door is closed the doorposts are outside the room used for bathing or relations, a Mezuzah is required.

Rebuttal (Aruch ha'Shulchan 15): The Gemara, Rambam and Poskim forbid relations in front of Seforim and Tefilin unless there are two covers, but it does not mention any problem with the Mezuzah! This is because the Torah commanded to have a Mezuzah and it commanded Peru u'Revu; it did not command to have other Divrei Kedushah in the room.

Shulchan Aruch (ibid. 4): A Beis ha'Kisei, tannery, bathhouse, or Mikveh is exempt for these are not dignified residences.

Question: Why is this reason required? One may not bring Divrei Kedushah into a place of filth or Ervah!

Answer (Aruch ha'Shulchan 5): One may bring Divrei Kedushah in if they are covered. These places are exempt from Mezuzah and it is forbidden to put a Mezuzah there even if it is covered.

Shulchan Aruch (ibid. 5): A Mezuzah in a filthy place, e.g. where babies are found, should be covered. A Mezuzah in a Tahor place should be exposed.

Taz (5): A Mezuzah should not be totally uncovered, for then 'Shakai' on the outside will be rubbed out (by touching it to kiss it). Rather, it should be covered with glass. A glass covering also precludes problems if occasionally the room is filthy or is used for bathing; we find that one may read Shma near excrement if it is covered by glass, even though the excrement can be seen. If the Mezuzah is at least 10 Tefachim off the ground it is a different Reshus, and no covering is needed; nevertheless, it is best to have a glass covering. An opaque covering is required to have relations there - "V'Lo Yir'eh Vecha Ervas Davar".