The variation of sexual motivation with duration of partnership is analyzed in data from a survey of German students. The sample of 1865 includes only students aged 19-32 who reported to be heterosexual and to live in a steady partnership. Main results are (1) sexual activity and sexual satisfaction decline in women and men as the duration of partnership increases; (2) sexual desire only declines in women; and (3) desire for tenderness declines in men and rises in women. Because these results are based on cross-sectional data, a longitudinal explanation is precarious. Individual differences in mating strategy associated with the probability of having a partnership of shorter or longer duration at the time of the survey may account for some part of the findings. This possibility set aside, post hoc explanations for the results as reflecting a modal time course of partnership are evaluated with regard to habituation, routine, gender role prescriptions, and polarization of roles. In addition, an explanation from evolutionary psychology is offered, entailing the following ideas: the psychological mechanisms of attachment in an adult pair bond have evolved from the parent-child bond. Due to this nonsexual origin, a stable pair-bond does not require high levels of sexual desire, after an initial phase of infatuation has passed. Nevertheless, male sexual desire should stay at a high level because it was selected for in evolutionary history as a precaution against the risk of sperm competition. The course of female sexual desire is assumed to reflect an adaptive function: to boost attachment in order to establish the bond.