Gender identity doesn't belong in elementary school
It is psychologically confusing to teach a child the biology of male and female in the animal kingdom and then imply that this only applies to every animal except humans.
Elementary age children play doctor, house, and other forms of imagination play as they learn the roles in society. They even play as other animals. This doesn't mean they identify as that animal in later years. Therefore we have no business pushing children to identify as the opposite sex simply because they played at it trying to understand it.
Let children be children and keep sex out of it.