The mitzvah of onah, the mitzvah of marital sex, raises the issue of the symmetry or asymmetry of the halakhically-defined sexual relationship. The Gemara discusses at length the husband’s obligation to have sex with his wife (at regular intervals or when she indicates that she is interested) based on the Biblical obligation of onah. However, while the Gemara takes it for granted that a wife must have sex with her husband (and one who regularly refuses to do so is termed a moredet, a rebellious wife, and can be divorced with cause), it never makes clear from where this obligation is