Gender based upon sex evolved as a way to guide people to having sex with members of the opposite sex. Such guidance was naturally selected because it produced more offspring than people who chose sexual partners irrespective of sex, wherein half the pairings could not reproduce. Gender is a part of the sexual phenotype that resides in the brain. For complex reasons, not everybody "hears" this guidance, and it is those people who have created the modern concept of gender identity. This modern concept will die out after a few generations because it cannot reproduce as effectively as those who seek the opposite sex.
Discussion
Gender is indeed a complex concept, and it is true that having sex with members of the opposite sex has historically been considered an important part of gender guidance. At the same time, it is important to recognize that there is a wide range of gender identities and expressions which exist beyond the binary of male and female, and that people should have the freedom to express their gender in whatever way makes them most comfortable.
Additionally, it is important to recognize that gender has traditionally been defined in a certain way, and that it is not always easy for communities to adjust to such changes. Ultimately, we must strive to create a society that is inclusive and supportive of all gender identities, and that recognizes and respects the right of individuals to express their gender in whatever way makes them most comfortable.