Cogenitor

The Enterprise crew makes first contact with the Vissians, a species with three genders. Trip meets one of the species' third gender, a Cogenitor, a being used for only breeding purposes. After learning how badly Vissians treat all Cogenitors, Trip attempts to help this Cogenitor, even though it directly interferes with Vissian culture.

Synopsis adapted from TVmaze (CC BY-SA 4.0)