The flower's unisexuality prevents

A unisexual flower has three whorls that include petals, sepals, the male reproductive structure (stamen), or the female reproductive structure (pistil).

Pollination by autogamy occurs when pollen grains are transferred from the anther of a flower to the stigma of the same flower.

Geitonogamy is the pollination of a flower by transferring pollen grains from the anther to the stigma of another flower of the same plant.