Gemara

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

If a man does so (puts his hand to his Ever to check himself), his hand is worthy to be cut off!

Question: Why are men and women different?

Answer: A woman is not Bas Hargashah.

This sensation in men causes emissions of semen.

Berachos 20a: R. Yochanan would sit next to the Mikveh in order that women should see his beauty when they emerged, and bear children beautiful like him.

48b: (The women who told Shaul where to find Shmuel said,) "... Lo Yochal ha'Am Ad Bo'o Ki Hu Yevarech ha'Zevach Acharei Chen Yochlu ha'Keru'im." (t/hey will not eat until Shmuel will make a Berachah Rishonah.)

Question: Why did they need to say all this?

Answer (Shmuel): They spoke at length to have more time to look at Shaul's beauty -- "mi'Shichmo va'Ma'alah Gavoha mi'Kol ha'Am."

Sukah 51b (Mishnah): On Motza'ei Yom Tov there was a great Tikun (for Simchas Beis ha'Sho'evah).

(R. Elazar): At first, Ezras Nashim was flat. They surrounded it with balconies, and enacted that women sit above, and the men are below.

(Beraisa): At first, women were inside and men were outside. They came to frivolity. They enacted that women sit outside and the men inside. They still came to frivolity. They enacted that women sit above, and the men are below.

Rishonim

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 inside), whereas men are Mazri'a externally.

Sefer ha'Chinuch (188): One may not get pleasure from any Ervah, i.e. relative, married woman or Nidah. The Torah forbids Kirvah (drawing close) in a way that could lead to Bi'ah. We may not do the abominations of Eretz Mitzrayim or Eretz Kena'an, e.g. a woman marrying a woman... A man may not wink, gesture or be frivolous with a woman, or smell fragrances on her, look at her for pleasure, except for one's wife. One may look at his wife even when she is Nidah, but not in places normally covered, lest he incite his Yetzer Ra. One may not look even at a woman's hair, hear her voice for pleasure, look at her colored clothing, or anything that will bring him to Hirhur. Yirmeyahu rebuked men who raised their voice to arouse women's Yetzer Ra for adultery. The Gemara discusses certain Chachamim who were lenient for the sake of a Mitzvah, e.g. R. Yochanan used to sit near the Mikveh, so women would bear children beautiful like him. Surely he did not look at them! It was clear to all that they intended l'Shem Shamayim, and that they were not affected by it, because they clung to Torah so much. This Lav applies to men and to women. Also a woman may not think about men other than her husband. All her desire should be for him. All Kosher Benos Yisrael do so.

Sefer Chasidim (614): A woman should not hear a man sing, just like a man may not hear a woman sing. Whatever he is commanded, she is commanded.

Poskim

Shulchan Aruch (YD 335:10): Regarding stomach illness, a man may not attend to a sick woman, but a woman may attend to a sick man.

Darchei Moshe (4): This is from Evel Rabsi 12. Why does it specify stomach illness? I explain that it is more lewd and disgraceful for a man to serve and clean a woman and such matters than vice-versa, for a man's Yetzer Ra is greater.

Taz (5): I explain that one with stomach illness needs to get up to eliminate often. If he serves her and engages often with her body, he will come to an erection. This does not apply to other illnesses. If he has a stomach illness, he lacks strength for an erection.

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. Why aren't we concerned for his Hirhur? The Terumas ha'Deshen (252) says that since he is sick, he is weak and his Yetzer Ra will not overpower. See the Taz and the Rambam in Nidah (13a).

Ha'ara'ah 3: The Ramban (Nidah 13a) forbids a woman to waste seed, even though she may sterilize herself. The Isur to waste seed does not depend on Peru u'Rvu. This seems unlike the Birkei Yosef, who says that we are not concerned for her Hirhur. Really, the Birkei Yosef holds that the Gemara is clearly like the Ramban, that a woman does not feel (in a way that would cause wasting seed) like a man does, because her Yetzer Ra is smaller. He cited the Ramban to teach that even though some argue with Tosfos, also according to them we are not concerned for her Hirhur.

Divrei Yatziv (EH 9): If one delays marrying off his daughter when she is a Bogeres, he is called a Rasha (Sanhedrin 76b). If one's Bogeres did not marry yet, he should free his slave in order to marry her to him (Pesachim 113a). Concern for Aveirah overrides the Isur Aseh to free one's slave. Perhaps there is more concern for Hirhur of a son (who did not yet marry) than for a daughter. However, Sefer Chasidim equates women to men. In Medrash Shmuel (13), R. Yehudah says that Benos Yisrael looked at Sha'ul's beauty and were not satiated with him. R. Yosi says 'according to you, they were like harlots! Isn't it true that just like a man may not get pleasure from looking at a woman who is not his, a woman may not get pleasure from looking at a man who is not hers?' The Maharsha (Berachos 48b DH Kedei) brings this. See Alshich (Shmuel 1:9:11-13?). Teshuras Shai (1:125) says that it is proper to arrange the Beis ha'Keneses so that women will not get pleasure from looking at men, for the Halachah follows R. Yosi (against even a majority). Also, 'isn't it true that' connote that all must agree 'that also women may not get pleasure', just R. Yehudah was not concerned for R. Yosi's initial words (according to you, they were like harlots). The Ramban says that if women were Benos Hargashah, they would be Chayav Niduy for checking extra, for this brings to Hirhur. The Me'iri explains that checking does not incite their Yetzer Ra. If it did, they would be forbidden, like men are. According to the Poskim that women are commanded like men, one must be equally concerned to marry them off promptly just like men. The Rambam (Hilchos Isurei Bi'ah 22:21) says that Hirhur comes only in a heart empty of Chachmah. If so, there is less concern for a man if he diligently learns Torah.

Divrei Yatziv (35): Kesuvos 64b says that we find that men hire women for harlotry, but not vice-versa. A man's Yetzer Ra is greater than a woman's. This is why the Shulchan Aruch (EH 21) details how men must avoid looking at women, but not vice-versa. However, surely also women have a Yetzer Ra and the Isur of looking at Arayos and Hirhur applies to them. The Torah equated women to men for all punishments. The Gemara says that the Da'as of women is light (Shabbos 33b, Kidushin 80b. I.e. they are prone to be enticed.) Indeed, R. Tam says that women have no Isur of Hargashah or wasting seed, but perhaps the Halachah does not follow him. It seems that there is an Isur Hirhur not dependent on wasting seed. Just like the Torah forbids Kirvah that leads to Bi'ah, such Hirhur is forbidden.

Question: R. Yochanan sat next to the Mikveh, so women would see his beauty and bear children beautiful like him. Why wasn't he concerned lest they have Hirhur?

Answer (Divrei Yatziv): R. Yochanan did not refer to physical beauty. This is not an attribute. R. Yochanan's eyelids covered his eyes (Bava Kama 117a. I.e. he was not physically beautiful.) Rather, he had a semblance of Yakov Avinu, i.e. the light of Kedushah radiated on him, and he illuminated in a dark room (Berachos 5b). A woman may not think about another man's beauty during Bi'ah!

Ben Yehoyada (Sukah 51b DH Hiskinu): At first, men and women were outside looking in at Simchas Beis ha'Sho'evah, and men saw women from the back. They enacted that the women will stand outside, so that the men will not look at the women. They would be embarrassed to turn around to look at them. Chachamim were more concerned for men looking at women (than vice-versa), for men often have Hirhur. There were still problems due to men passing by the women when they entered, and women looking at the men, so they enacted to totally separate them.

Divrei Yatziv, citing Tehuras Shai (125): The balcony in Ezras Nashim was all around, so (some) women would see the men's faces. We cannot learn from there, for there was no alternative. Before they made the balcony, men and women mixed and came to frivolity, and even so they did not stop women from coming! I say that there is no concern if women look at Chasidim who cling to Hash-m, just like there was no concern regarding R. Yochanan. In every generation women come to get Berachos from Tzadikim and see them. It has always been the custom that women visit Tzadikim. The Bavli (Berachos 48b) did not bring R. Yosi's opinion, for we hold like R. Yehudah.