Why must gender identity always follow biological sex?

What if God made someone male, but their biological body developed as female? Why wouldn’t that be possible?

Why do some christians always assume a person must conform to their biological body, rather than considering that the gender they consciously feel could actually be their true identity?

If consciousness persists beyond the body, it actually makes more sense that the eternal conciousness is the true identity, and the temporary biological form is a manifestation corrupted by "the Fall" as the story goes.