Gemara

(Beraisa - R. Meir): If a man was Mekadesh a woman on condition that he is exempt from giving to her food, clothing or Onah (the proper frequency of Bi'ah), the Kidushin takes effect and his stipulation is void;

R. Yehudah says, in monetary matters the stipulation is valid.

Gitin 84a (Abaye): If a man said 'this is your Get on condition that you eat pork', this is a stipulation that cannot be fulfilled (it is void).

(Rava): The stipulation is valid. It can be fulfilled, even though it is forbidden.

Bava Metzi'a 94a (Mishnah): An unpaid watchman may stipulate to be exempt from swearing...

Question: The stipulation is contrary to Torah. It should be void!

Answer #1: Our Mishnah is R. Yehudah, who says that such stipulations are valid.

Answer #2: Really, our Mishnah is R. Meir. Here, the stipulation works because the watchman never obligated himself.

Rishonim

Rif (Kidushin 25b): If a man was Mekadesh a woman on condition that he is exempt from giving to her food, clothing or Onah, R. Meir says that the Kidushin takes effect and his stipulation is void. R. Yehudah says, a stipulation about monetary matters is valid.

Ran (DH Garsinan): One can pardon monetary matters, but (deprivation of) Onah is physical pain. It cannot be pardoned.

Rambam (Hilchos Ishus 6:10): If a man was Mekadesh a woman on condition that he is exempt from giving to her food, clothing or Onah, the stipulation takes effect regarding food and clothing, for they are monetary. The stipulation about Onah is void, for the Torah obligated you. She is Mekudeshes, and you are obligated to fulfill Onah. You cannot exempt yourself through a stipulation. The same applies to everything similar.

Mordechai (Bava Metzi'a 369): The Yerushalmi says that a monetary stipulation contrary to Torah is valid, e.g. Kidushin on condition that she will have no rights to food, clothing, or Onah. A non-monetary stipulation contrary to Torah is void, e.g. she will not fall to Yibum. We infer that Onah is a monetary stipulation. This is because it can be pardoned.

Mishneh l'Melech (DH Ach): Maharik (10) says that R. Tam learns from the Yerushalmi that Onah is a monetary stipulation. R. Yehudah holds that all three stipulations are valid. He says 'any monetary stipulation is valid' to exclude a stipulation about Yibum. However, Tosfos holds like Rashi that Onah is a non-monetary stipulation, without bringing another opinion. The Ritva says that Onah is a monetary stipulation because it is a bodily pleasure. An early version of Rashi (cited in Shitah Mekubetzes Sof 56a DH v'Zeh) says 'people are apt to pardon money, but she did not pardon Onah, which is bodily pain.' According to this, if she explicitly pardoned it, the stipulation is valid. Regarding money, we assume that he stipulated that she pardon him. Since people normally do not pardon pain, we assume that he intended to exempt himself. Also a stipulation 'on condition that you have no claim of food and clothing' is Batel.

Mishneh l'Melech (DH Od): A woman can permit her husband to withhold Onah, but this is not through pardon. Rather, as long as she does not request Onah, he may refrain, for he has her permission. (The Mitzvah is to satisfy her desire.) Clearly, if she requests later, he is obligated.

R. Akiva Eiger: The Ra'anach (2:43) was unsure whether or not he is obligated after she requests later.

Rambam (12:7): If a husband stipulated that she has no rights of Onah from him, it is Batel, and he is obligated. This is because he made a non-monetary stipulation contrary to Torah. (Note: it seems that here the Rambam discusses a stipulation made during the marriage.)

Question (Tosfos 56a DH Harei): R. Meir says that the stipulation is Batel because it is contrary to Torah. We must say that he made a Tanai Kaful (if I am exempt, there is Kidushin; if I am not exempt, there is no Kidushin. If not, R. Meir would say that the stipulation is Batel because it was not doubled!) If so, why is she Mekudeshes when the stipulation is not fulfilled?

Answer (Tosfos): If we did not learn from Benei Gad and Benei Reuven, we would have said that a stipulation cannot Mevatel an action, even if it was not fulfilled. We have no source to say that a stipulation contrary to Torah can Mevatel an action.

Question: A watchman may stipulate to be exempt from his obligations, because he never obligated himself. The same should apply to Kidushin on condition to be exempt from food and clothing!

Answer #1 (Rashi Bava Metzi'a 94a DH Afilu): Kidushin is different, because he stipulated after he was Mekadesh her, and there cannot be partial Kidushin.

Objection (and Answer #2 - Tosfos, ibid.): R. Meir requires every stipulation to precede the action! Rather, because the Torah gave different laws for different watchmen, one can stipulate what his obligation will be. Alternatively, the Torah obligates a borrower and a paid watchman (more than one who guards for free) because the former gets all the benefit, and the latter is paid. Therefore, when they did not obligate themselves they are exempt.

Rashba (brought in Shitah Mekubetzes Sof 56a): R. Meir holds that even though he said 'Al Menas', a stipulation against Torah is like words said in jest. He intended that the law will not apply. R. Yehudah argues only about the intent of his words. Regarding a monetary stipulation, he says that he intended that she pardon him. A stipulation about pain cannot be pardoned, so it is mere jest.

Ketzos ha'Choshen (60:3): The Tur says that just like one cannot obligate himself in something unlimited, one cannot pardon something unlimited. This is difficult, for the Rashba says that R. Yehudah validates a stipulation regarding food and clothing because she pardons it! It seems that just like one can obligate himself in something unlimited, amidst (the Simchah of) marriage, one can pardon something unlimited amidst marriage.

Poskim

Shulchan Aruch (EH 38:5): If a man stipulated at the time of Kidushin that she will have no (rights to) food and clothing, his stipulation is valid. If he stipulated that he will have no obligation of Onah, the stipulation is Batel and he is obligated.

Beis Shmuel (10): Chelkas Mechokek says that if he said 'on condition that there are no (rights to) food and clothing in the Kidushin', the stipulation is Batel.

Question: If one was Mekadesh 'on condition that you eat pork', all should agree that the stipulation is void, for it is contrary to Torah!

Answer #1 (Drishah): There, she can eat and be lashed. The purpose of marriage is Onah. If this is pardoned, what is the purpose of Kidushin?!

Answer #2 (Beis Shmuel 12): A stipulation to eat pork does not necessitate uprooting Torah law. She can refuse to eat, and the Kidushin or Get will be invalid! Kidushin on condition that there are no rights to Onah surely uproots Torah law.

Chelkas Mechokek (77:3): If a man wants to divorce his wife because she has a blemish and is repulsive to him, perhaps R. Gershom did not forbid to divorce b'Al Korchah (against her will) in such a case. It seems that one must give to her food and clothing. He initially pardoned the blemish and obligated himself to give to her a Kesuvah, so the same applies to stipulations of the Kesuvah. However, regarding Onah, which is physical pain (for him to fulfill it when she is repulsive to him), even if he pardoned once, it is not permanent.

Ketzos ha'Choshen (73:3 DH v'Ayen): A stipulation contrary to Torah does not take effect, even if it includes valid stipulations. R. Yehudah says that a stipulation to exempt oneself from Onah is void, even if it was with stipulations to exempt himself from food and clothing, which take effect.