Sex is critical to marriage. Yet, there are several reasons to expect spouses to experience declines in the desire for sex over time, and the rates of any declines in sexual desire may differ for men and women. We used two multi-wave, longitudinal studies to test whether male and female members of newlywed couples experienced different rates of change in sexual desire, whether any such changes were accentuated by childbirth, and whether any such changes had implications for marital satisfaction. In both studies, spouses provided multiple reports of sexual desire, marital satisfaction, and childbirth. Results demonstrated that women's sexual desire declined more steeply over time than did men's sexual desire, which did not decline on average. Further, childbirth accentuated this sex difference by partially, though not completely, accounting for declines in women's sexual desire but not men's. Finally, declines in women's but not men's sexual desire predicted declines in both partners' marital satisfaction. These effects held controlling depressive symptoms and stress, including stress from parenthood. The current findings offer novel longitudinal evidence for sex-differentiated changes in sexual desire and therefore suggest an important source of marital discord.