Opuntia robusta has hermaphroditic, dioecious, and trioecious populations. To enhance our understanding of this breeding system diversity, we compared the reproductive output of males, females, and hermaphrodites in a trioecious population using field evaluations, controlled crosses, and progeny tests. Unisexuals were fully sterile in one sex function. Hermaphrodites were fully fertile for both functions. Consistent with the sex-allocation theory, unisexuality increased the quality and quantity (in males) of the gametes of the functional sex, relative to those of hermaphrodites, probably explained by maternal and paternal effects. The increase was higher in males than in females, suggesting a more expensive female function. Theoretically, this disproportional increase is required for unisexuals to invade a hermaphroditic population with prior selfing, negligible pollen discounting, and undetectable inbreeding depression, features found in O. robusta, therefore helping to explain dioecious populations. However, in the study population, the actual seed output of females was lower and had a higher variance than that of hermaphrodites, which also reproduce through pollen. Unisexuals are unlikely to be maintained by their actual reproductive output in this pollen-limited environment. Hermaphrodites may persist in this population by producing their seeds autonomously and by reducing interspecific fertilization by prior selfing and ovule discounting.