As many have noted, you have misrepresented sex and gender. Sex is biological, gender is a social identity. In that regard, gender can be changed as much as my political identity can be changed, as it is separate from a biological or ethnic background. Gender norms and traditions are derived from sex differences, but those norms are not ironclad and are shaped by the culture of society. A tomboy is a girl biologically, but she assumes many of the gender norms of a male. In modern society some individuals find it necessary to take it a step further and be referred to that pronoun because of their intense identification with that identity. It's up to you to decide whether to respect that.