The adaptive significance of sex change and other sex allocation patterns in hermaphrodites is often explained by relying on the principle that energy allocation to the female function increases with increasing resources for reproduction. However, existing sex allocation models do not cover the alternate gender-role change, sporadically found in outcrossing spawners showing simultaneous hermaphroditism. Here, we established a new theoretical framework to understand their reproductive patterns without assuming the individual variation of resource availability. The model addresses a bidirectional gender-role change shown by an intertidal holothurian, on the assumption that natural selection favors a parental genotype that maximizes the invasion fitness of offspring population consisting of multiple reproductive schedules. Mathematically, our scheme is a natural extension of the classical Fisher’s principle based on negative frequency-dependent selection, and has a high capability to explain a strongly biased operational sex ratios frequently found in both dioecious and hermaphroditic species.