This paper has two aims: first, to review work addressing the functional significance of variation in sexuality across the women's menstrual cycle and its implications for an understanding of human sexual nature; second, to illustrate the more general use of adaptationism in sex research. Adaptationism provides a method for recognizing adaptations, traits that evolved because they bestowed reproductive advantages upon their owners. The telltale sign of adaptation is its special design for a particular function. In recent years, evolutionary psychologists have explored changes in women's sexuality and standards of male attractiveness across the menstrual cycle. Evidence provisionally supports the idea that these changes constitute special design for the function of obtaining genetic benefits through mating with men other than primary partners.