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The Gemora explains that Rabbi Yehudah holds that one can make a condition modifying the obligations stipulated by the Torah regarding monetary law.
This would explain why Rabbi Yehudah holds that the condition is valid when he stipulated that he does not owe her support or clothing; however, why is it valid when he stipulates that he will not have marital relations with her? This is not a monetary law!?
Rashi, because of this, writes that the husband remains obligated to have marital relations with her, for this is not a financial right. Depriving a wife from relations would cause her physical distress and therefore the condition is void.
The Mishnah Lamelech challenges this from a Gemora which states that one can say to his fellow, “Hit me and you will be exempt.” Evidently, one can waive physical anguish! Furthermore, we find that a woman can release the husband from his marital relations!?
Some answer that Rashi himself, cited in the Shitah Mikubetzes in Kesuvos (56a), states that the condition is void, for we assume that a woman will not waive her rights regarding anything which causes physical anguish; however, if she explicitly forfeits those rights, they are forfeited.
Rabbeinu Chananel holds that a man may stipulate on marital relations, and a wife can waive her rights to it as well. This is because the pleasure of relations belongs to her and it would be regarded as a financial right.