Outbreeding devices

When unisexual male and female flowers are present on different plants, the condition is called dioecious, and it prevents both autogamy and geitonogamy.

Papaya is dioecious: male and female flowers are on different plants.
So neither autogamy (same flower) nor geitonogamy (another flower of the same plant) is possible.
Maize, cucumber and castor are monoecious; this prevents autogamy but not geitonogamy.

Autogamy is the transfer of pollen from the anther to the stigma of the same flower.
Geitonogamy is the transfer of pollen from the anther of one flower to the stigma of another flower of the same plant.
Xenogamy is the transfer of pollen to the stigma of a flower on a different plant.
A unisexual flower has either stamens or carpels, so pollen can never reach the stigma of the same flower; autogamy is prevented.
If both male and female flowers are present on the same plant (monoecious condition, as in castor and maize), pollen from a male flower can still reach a female flower of the same plant, so geitonogamy is not prevented.
Both autogamy and geitonogamy are prevented only when male and female flowers are borne on different plants (dioecious condition, as in papaya).
So unisexuality of flowers prevents autogamy, but not geitonogamy.