That's... that's the point. I'll refer to you however you want to be referred (inb4 helicopter joke). There's nothing more to it. Just accept other people's preference and move on with your day. It's that simple.
For instance - it was once considered perfectly masculine behavior for men to wear pantyhose, high heels and makeup, and to kiss each another on the mouth. Now, those traits would likely be considered feminine (or at least, not masculine.)
Does that mean men were formerly somehow more biologically female, because they expressed what we would consider feminine traits? Obviously not. Was their gender female, then? Not in the context of their own culture, they were men being what men were considered to be.
It does mean the constructs of "masculinity" and "femininity" are not innately biological, and that the correlation between sex and gender is mostly a manifestation of cultural norms, not biological imperative.
All of the confusion in your comment seems to stem from the assumption that sex and gender are a priori the same. You may find Wikipedia's article on gender to be useful[0].
Visible light is a spectrum, yet we still use labels for distinct colors, despite "color" being arbitrary (a "social construct".) Labels can still be useful for communicating concepts and establishing a common ground, even if they're imprecise, or if different cultures don't even agree on the difference between "blue" and "green."
>Do a Ctrl+F of the linked article for "Gender" and "Sex" and then tell me that the research isn't also confused because everyone is confused all of the time, especially people who are "confused".
Everyone isn't confused all the time. People do disagree, though, and society is still working out just what gender means in the modern day.