Gemara

(Beraisa): If a man divorced his wife on condition that she ascends to Heaven, the Get is invalid;

R. Yehudah ben Teima says, such a Get is valid. The rule is, if a Tenai cannot be fulfilled, he merely taunts her with words. The Tenai is void, and the Get is valid.

(Rav Nachman): The Halachah follows R. Yehudah (ben Teima).

(Rav Nachman bar Yitzchak): A Mishnah teaches that a Tenai that can be fulfilled is valid. This implies that an impossible Tenai is invalid, like R. Yehudah.

(Abaye): Also a Tenai 'on condition that you eat pork' is impossible (it is void).

(Rava): The Tenai can be fulfilled, even though it is forbidden.

Question (against Rava - Beraisa): 'On condition that you have Bi'ah with Ploni', is a valid Tenai in a Get. 'On condition that you will not have Bi'ah with my or your father', is valid. We are not concerned lest she will (the Get is valid).

The Beraisa omits a Tenai 'that you will have Bi'ah with my or your father'!

Answer (Rava): A Tenai to eat pork is valid, for she can do this (albeit she will be lashed for it). A Tenai to have Bi'ah with Ploni is valid, for she can bribe him. But even if she is willing to sin, she cannot get her father or father-in-law to sin!

Rava explains R. Yehudah's rule (any Tenai that cannot be fulfilled is void) to include a Tenai to have Bi'ah with her father or father-in-law. 'Such a Get is valid' excludes the case of eating pork. (She is divorced only she fulfills the Tenai.)

Abaye explains that the rule (a Tenai that cannot be fulfilled is void) includes a Tenai to eat pork. 'Such a Get is valid' excludes Bi'ah with Ploni.

Bava Basra 126b (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 Tenai is void;

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

Kidushin 19b (Beraisa - R. Meir): If one sold his daughter and stipulated that the master will not do Yi'ud (marry her himself, or to his son), the Tenai is valid;

Chachamim say, he may do Yi'ud, for a Tenai contrary to Torah is void.

Question: R. Meir holds that Kidushin on condition to be exempt from giving food, clothing or Onah is an invalid Tenai!

Answer (Chizkiyah): Here R. Meir expounds "for an Amah" - a sale is valid even if it is only to be an Amah (and cannot result in Yi'ud).

Menachos 109a (Mishnah): If one vowed to bring an Olah on condition to offer it in Beis Chonyo (a place in Alexandria where certain Kohanim offered Korbanos), he must offer it in the Mikdash. If he offered it in Beis Chonyo, he was Yotzei.

Rishonim

Rif: The Halachah follows R. Yehudah ben Teima.

Rambam (Hilchos Ishus 6:8): If one stipulated about something possible but forbidden, e.g. 'if you will eat pork this is your Get. If you will not eat pork, it is not your Get', and then he gave to her the Get, the Tenai is valid. She is divorced only if she transgresses and eats. This Tenai is not contrary to Torah, for she can refrain from eating and not be divorced.

Rambam (9): Any Tenai contrary to Torah is void, except for a monetary Tenai. E.g. one was Mekadesh or divorced on condition that he will merit something that the Torah did not entitle him to or withheld from him, or exempted himself from a Torah obligation. His Tenai is Batel, and his action was valid. He did not exempt himself or merit what he sought.

Rambam (Hilchos Avadim 4 16): If one sold his daughter on condition that the master not do Yi'ud, he can do Yi'ud. The Tenai is contrary to Torah; it is Batel.

Rosh (Gitin 9:2): The Halachah follows Rava.

Question (Tosfos Kesuvos 56a DH Harei): R. Meir invalidates the Tenai because it is contrary to Torah. Surely, the Tenai was Kaful (if I am exempt, there is Kidushin; if I am not exempt, there is no Kidushin), for if not R. Meir always invalidates it! If so, why is she Mekudeshes when the Tenai is not fulfilled?

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

Question: A vow to bring a Korban on condition to offer it in Beis Chonyo should be Batel, for it is contrary to Torah!

Answer (Tosfos): A Tenai contrary to Torah is Batel only if he intended to uproot Torah law. There, he did not intend to uproot. He thought that there is a Mitzvah in Beis Chonyo!

Mordechai (Kesuvos 213): Regarding food and clothing (and Onah - Bach deletes this from the text), a Tenai helps.

Mordechai (Bava Metzia 369): The Yerushalmi validates a monetary Tenai contrary to Torah, e.g. Kidushin on condition that he is exempt from food, clothing, and Onah. A non-monetary Tenai contrary to Torah is void, e.g. she will not fall to Yibum. Onah is a monetary Tenai, for it can be pardoned.

Poskim

Shulchan Aruch (EH 38:5): If a man stipulated at the time of Kidushin that he is exempt from food and clothing, his Tenai is valid. If he stipulated that he is exempt from Onah, the Tenai is Batel and he is obligated.

Question: If one was Mekadesh 'on condition that you eat pork', all should agree that the Tenai 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 Tenai to eat pork will not necessarily uproot Torah law. She can refuse to eat, and the Kidushin or Get will be invalid! Kidushin without rights of Onah surely uproots Torah law.

Shulchan Aruch (118:14): A Tenai in the Kesuvah not to marry another wife during the marriage is not contrary to Torah.

Be'er Heitev (9): Even if he will be childless for 10 years, he can divorce her and remarry. A Tenai not to marry another wife and not divorce her is Batel.

Shulchan Aruch (143:12): A Tenai about something possible but forbidden, e.g. to eat pork, is valid. It is not contrary to Torah, for she can refrain from eating and not be divorced. This refers to an Isur she must do. If it depends on others, e.g. to have Bi'ah with his father, is not a Tenai, for his father will not agree to sin.

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