The adult male accessory gland in insects is an internal reproductive organ analogous to the mammalian prostate, and secretes various components in the seminal fluid. Products of the accessory gland in the fruit fly Drosophila melanogaster are known to control reproductive behaviors in mated females, such as food uptake, oviposition rate, and rejection of re-mating with other males, all of which increase male reproductive capacity. Production of larger amounts of accessory gland products is thus thought to result in higher male reproductive success. The epithelium of the Drosophila accessory gland lobe is composed of a unique population of binucleate cells. We previously predicted, based on measurements of cell size in mono/binucleate mosaic accessory glands, that binucleation results in a higher plasticity in cell shape, enabling more effective ejection of seminal fluid. However, the actual effect of binucleation on ejection of seminal fluid or reproductive capacity remained unclear, as we were unable to generate an organ with uniformly mononucleate cells. In the present study, we generated organs in which most of the epithelial cells are mononucleate by manipulating aurora B or fizzy-related to block binucleation. Mononucleation resulted in a less elastic accessory gland lobe, which decreased ejection volume and the oviposition of mated females; these effects were particularly pronounced over the long term. These results suggest that binucleation in accessory gland epithelial cells contributes to higher plasticity in the volume of this organ, and enhances male reproductive success through enabling ejection of larger amounts of seminal fluid.