Gemara

Nedarim 20b: (R. Yochanan ben Dehavai): The ministering angels told me four things... Lame people result from parents who had Bi'ah Lo k'Darkah (in the anus)... Deaf people result from parents who spoke during Bi'ah.

Question: Eima Shalom said that her children were exceedingly beautiful because her husband (R. Eliezer) did not have relations with her at the beginning or end of the night, only at midnight.

I asked him (at the time, why he has relations only at midnight). He said that it is lest he think about other women, which would make the children like Mamzerim.

Answer: One may talk only about matters of Bi'ah.

Chagigah 5b - Question: Rav Kahana went under Rav's bed. He heard Rav converse and joke with his wife before Bi'ah.

Answer: One may talk with her to appease her if this is needed. R. Yochanan ben Dehavai discusses when this is not necessary.

Nedarim 20b (R. Yochanan): This is R. Yochanan ben Dehavai's opinion. Chachamim say, a man may do whatever he wants with his wife;

(Ameimar): R. Yochanan ben Dehavai heard these matters from 'ministering angels', i.e. Rabanan. If actual angels had told him, R. Yochanan would not have ruled against him. The angels understand the formation of fetuses better than we do!

"Do not follow your hearts" forbids thinking about another woman during Bi'ah.

(Ravina): We need to teach this only for the case when also the other woman is his wife.

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

...He hates her (and therefore thinks about another woman), Temurah (he thought that this was a different wife of his), Irbuviya (confusion)...

Nidah 71a (Rav Chama bar Chanina): "Sachar Peri ha'Beten" teaches that if a man delays ejaculation while on his wife's stomach, in order that she will be Mazri'ah first, Hash-m rewards him with fruit of the stomach (sons).

Rishonim

Rambam (Hilchos De'os 5:4): It is not proper for a Chacham to have relations in the beginning of the night when he is satiated, nor at the end of the night when he is hungry, rather, in the middle of the night.

Hagahos Maimoniyos (2): Tosfos says that at the beginning of the night we are concerned lest he think about women he saw during the day or that he hears now. The latter concern also applies to the end of the night.

Rambam (10): It is the way of Kedushah to have Bi'ah only in the middle of the night.

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

He thinks about another woman at the time...

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:

...He intends to have Bi'ah with his wife Rachel and had Bi'ah with his wife Leah.

Rosh (DH b'Mili): One may discuss matters of Bi'ah at the time of Bi'ah in order to increase his desire.

Rosh (DH Bnei Irbuvya): Irbuvya is when he is not sure which with which of his wives he had Bi'ah.

Poskim

Shulchan Aruch (EH 25:2,3): One should not be frivolous with his wife, nor foul his mouth with nonsense, even between them. It says "U'Magid l'Adam Mah Secho" - one will be judged even for light conversations between him and his wife. One should not talk with her at the time of Bi'ah or beforehand, lest he think about another woman. If he did talk and had Bi'ah immediately afterwards, the above verse applies. One may talk with her about matters of Bi'ah, to increase his desire, or to appease her if he had been angry at her.

Beis Yosef (DH v'Al): The Gemara answered that one may talk only about matters of Bi'ah. It did not say that R. Eliezer spoke to her before Bi'ah. This connotes even beforehand, one may not talk (about other things).

Ra'avad (cited in Beis Yosef OC 240 DH Aval): From when he asks her to have Bi'ah, he may not discuss other things, lest he think about another woman. The Gemara allows appeasing her before Bi'ah if necessary. This implies that if he does not need to, he should not talk even before Bi'ah.

Shulchan Aruch (OC 240:2): During Bi'ah he may not think about another woman, even if both of them are his wives.

Shulchan Aruch (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 hates her, Temurah, Irbuvya...

Kaf ha'Chayim (34): Temurah is when he thinks about another woman (a Yisraelis or a Nochris), and that woman thinks about him when she has Bi'ah. This switches the Neshamos, and can make the child a heretic.

Question: The night that Yakov married Leah, he thought that he was with Rachel. Tosfos says that Reuven was conceived from that Bi'ah!

Answer #1 (Aserah Ma'amaros, cited in Magen Avraham 9): Indeed, this is why Reuven did not receive a share in Eretz Yisrael, only in Ever ha'Yarden.

Answer #2 (Magen Avraham): Heaven forbid that one of the Shevatim was a rebel! We must say that the problem is only when he saw Plonis on a bed, and later thought that he was with Plonis. Here, Yakov saw Leah and intended to have Bi'ah with her, just he erred about her name.

Answer #3 (Kaf ha'Chayim 33): Yakov knew that Rachel was a Tzadekes and was concerned lest she give the Simanim to Leah, and he know that his first Bi'ah would produce a child ("Reishis Oni Atah"). Therefore, he intended for Bi'ah with whomever it is, to avoid the problem.

Answer #4 (heard from Rav Uzi'el Milevsky): Yakov's sole intent was to father Shevatim through his Zivug. When Yakov rebuked Leah for tricking him, she replied that he said 'I, Esav your Bechor' to receive the Berachos (Tanchuma Vayetzei 11). I.e. through this he received what was destined for Esav, including Esav's Zivug, i.e. herself. Therefore, he was not mistaken.

Kaf ha'Chayim (38): Irbuvya is when he is thinking about another woman, or she is thinking about another man.

Shulchan Aruch (ibid.): Some say that R. Eliezer 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.