Gemara

(Rav Avira): Yisrael were redeemed from Mitzrayim in the merit of the righteous women of that generation.

They would take food and water to their husbands in the field, wash and anoint their husbands, give them to eat and drink, and cohabit with them in ridges between the fields.

"If you will lie between the ridges..." in the merit of lying between the ridges, Yisrael merited the spoils of Mitzrayim - "dove's wings covered with silver, and its pinions with fine gold."

Nedarim 20b: R. Eliezer would rush Bi'ah as if a Shed (demon) was forcing him. He uncovered a Tefach and left a Tefach covered.

(R. Levi): "I will purge from you those who rebel against Me" refers to children who result from one of the following improper Bi'os:

She fears to tell him that she does not desire Bi'ah now, he forced her, he hates her (and therefore thinks about another woman), he was excommunicated, Temurah, he was quarreling with her, he was drunk, he resolved to divorce her, Irbuviya (confusion), or she verbally requested Bi'ah.

Question: R. Shmuel bar Nachmani said that if a woman requests Bi'ah, the resulting children will be greater than the elite of Moshe's generation!

Moshe sought (for the 70 elders) "Nevonim" (people who understand a matter from other matters). The elders picked were not called Nevonim.

Yisachar was conceived the night that Leah told Yakov to come to her tent, and it says "from descendants of Yisachar, who have Binah"!

Answer: It is proper to show her desire (without explicitly requesting Bi'ah).

Rishonim

Rambam (Hilchos Isurei Bi'ah 21:9): It is Midas Chasidus to sanctify oneself at the time of Bi'ah. He should not deviate from normal conduct. The matter is solely for the sake of reproduction.

Rambam (12): Chachamim forbade Bi'ah in the following cases:

He thinks about another woman at the time; he was drunk; he was quarreling with her; he hated her; it is against her will, but she fears (to refuse) him; one of them was excommunicated; he resolved to divorce her.

If he had such Bi'ah, the children will be improper. In some cases they will be brazen. In some cases they will be rebellious.

Rambam (13): Chachamim also said that the following Bi'os produce rebellious children who will be weeded out through afflictions of exile:

She is brazen and verbally demands Bi'ah; he entices a woman to have Nisu'in; he intended for Bi'ah with one wife, and had Bi'ah with another wife.

Perush ha'Rosh (Nidah 20b DH Megaleh): R. Eliezer intended to have shame and fear. Alternatively, he intended to avoid getting pleasure. This is like R. Yosi, who did Yibum through a sheet to avoid getting pleasure.

Gra (OC 240:26): One might have thought that it is praiseworthy to avoid pleasure only regarding Yibum, to be concerned for Aba Sha'ul's opinion that it must be Lishmah. We learn that it is proper regarding any wife.

Ran (DH k'Mi): He would rush like one forced by a Shed.

Rashi (DH Bnei Irbuvya): Irbuvya is when he is unsure with which woman he had Bi'ah (Rosh), she had Bi'ah with many men (Ran), or it is a Safek nine month baby from her first husband, or a seven month baby from her latter husband.

Rashi (Shmos 38:8, based on Medrash Tanchuma Pekudei 9): In Mitzrayim a woman would bring a mirror to her husband, see both of them in the mirror and entice him with words, saying 'I am prettier than you.' This aroused his desire to have Bi'ah with her. Hash-m desired the mirrors for the Kiyor.

Poskim

Shulchan Aruch (EH 25:2): One should not intend for his own pleasure, rather, like one who pays a debt. He is obligated in Onah (Bi'ah at regular frequency, based on his occupation) and Peru u'Rvu, and that his child should engage in Torah and fulfill Mitzvos. He should have Bi'ah only with her consent. If she does not, he should appease her until she does.

Shulchan Aruch (OC 240:3): "I will purge from you those who rebel against Me" refers to children who result from one of the nine improper Bi'os:

He forced her; he hates her; one of them was excommunicated, Temurah (he thought that this was another woman); Moredes (she does not desire him); one of them was drunk; he resolved to divorce her, Irbuvya, or she verbally requested Bi'ah.

Beis Yosef (DH v'Chol): The Ra'avad says that men who have such Bi'ah are called rebels. The child did not sin! However, perhaps children born from such Bi'ah are prone to rebel.

Prishah (EH 25:31): One may tell a woman that he will marry her and divorce her, and Bi'ah is permitted. However, it seems that this is permitted only if he told her before marrying her.

Magen Avraham (8): The Tur permits if he hates her, as long as he does not hate her at the time of Bi'ah.

Magen Avraham (10): Moredes is forbidden even if she consents at the time of Bi'ah.

Mishnah Berurah (18): We are concerned for intoxication, but not for merely drinking a Revi'is.

Mishnah Berurah (21): One must be careful about all of these matters even when she is pregnant or is too old to give birth. One should be careful not to have Bi'ah with his wife when she is sleeping.

Shulchan Aruch (8): He should have Bi'ah with fear and awe.

Kaf ha'Chayim (58,64): He should picture the letters of the Torah. Even though he may not say Divrei Torah with unclean hands, he may think. She should think about a great Chacham and his deeds, and desire that there be more like him in Yisrael. One should say beforehand that he has Bi'ah l'Shem Hash-m, l'Shem Mitzvas Onah, and to draw a Neshamah Kedoshah.

Shulchan Aruch (ibid.): R. Eliezer would expose a Tefach, cover a Tefach, and resembled one forced by a Shed. I.e., he was in fear and awe like one forced by a Shed. Some explain that he would not enter the Ever all the way, in order to minimize his pleasure. It was as if he was forced against his will to do the act by a Shed. Some say that he would immediately cover his wife after the act, i.e. he did not delay, like one who was frightened by a Shed.

Question: One who delays during Bi'ah so that his wife will be Mazri'ah first is rewarded with sons!

Answer (Ra'avad, cited in Magen Avraham 21): If one knows that he can delay and not think about another woman, he should do so. If not, he should be quick to save himself from sin, and Hash-m will reward him for this.

Kaf ha'Chayim (59): In any case one must not withdraw until all drops of semen have left, lest he stumble in Zera l'Vatalah.

Shulchan Aruch (ibid.): Some say that he required his wife to wear a Sinar (undergarment) during the act, and he would uncover only a Tefach, to minimize his pleasure. All of these explanations are true. A Ba'al Nefesh (a spiritual person) must be careful about all of them.

Beis Yosef (DH Kosav ha'Ra'avad): A reason for this was Tzni'us. However, his main reason was to guard himself from thoughts of another woman.

Kaf ha'Chayim (61): The Zohar and Ari Zal hold like the first explanation (he was in fear like one forced by a Shed). There is a slight Isur in all the others.