Sex, ethnicity, weight, height, and age are physical properties of humans. They can be pretty easily measured and are nearly universally agreed upon. Gender, race, political affiliation and the like are far more fluid and harder to define. Identity is a hodgepodge of both rigid definitions and fluid characteristics that change over time, among societies, and within a person's own mind and body.
Unfortunately, we don't get much opportunity to discuss these kinds of things meaningfully outside of academia (and those discussions are often stilted, confusing, and stuffy). So when some stupid story like Rachel Dolezal or Bruce Jenner is the best chance we have, and even then we don't have a very good vocabulary to talk about it, you get this thread as a result.
I think I find myself squarely between the social constructivist camp Ph and wakephan describe and the Junebug/ELC mental illness camp.
I guess I just try to be sympathetic and kind to humanity in all forms, even people I have a really hard time identifying with.