A first draft of some gender theory for facebook:
"People say they don't understand transgender issues. Here's a primer, using only things you already believe (probably):
People can be straight, or gay, or bi. Everyone knows this. You could call orientation 'seeking behaviour': it's about what you find attractive. Many animals also do 'display behaviour'. Bowerbirds are an example: males build elaborate displays out of whatever they can find.
Humans do all sorts of display, with items, with their bodies, with the consequences of behaviour on their bodies (tattoos, clothing, hair, footbinding, vocal tone {way more cultural than you'd expect}, languages, social media profiles, plastic surgery). The kinds of display vary across cultures and times, but you won't find a culture where people don't display themselves somehow.
Now, let's imagine you're a boy, in a culture where women conceal their whole body (for whatever reason). You go through puberty, you're heterosexual, and you have no idea what women look like without a concealing robe. You would still learn to be attracted to women, as your biology points you that way, but you'd have nothing to be attracted to except external behaviour.
It is to say, there's an ultimate evolutionary purpose to your seeking behaviour, in that finding someone to reproduce with is how that works, but you have to infer reproductive capacity from arbitrary cultural signals. Those signals are what we call 'gender'.
So: being transgender is exactly like being gay. Analogous biology causes analogous 'not-matching-sex-reproductively' behavioral desire, and many desires held by trans people nowadays are 'display behaviour' desires like 'wanting a same-sex partner' is a 'seeking desire'.
It's also worth mentioning that, prior to modern medicine, most trans people would still be fertile, and, given that orientation and gender are separate things, we should expect being trans to have less of a negative effect on your number of children than being gay.
(If you're like 'but surely these people are unattractive and weird, I ask you to consider whether you've ever heard of people being into gender-nonconformity, and then have you listen to My Chemical Romance and as many K-pop boybands as you can get through).
So, if anything, the 'natural' percentage of people who would be some variety of transgender should be higher than the percentage who're gay, if both ways of being weren't disapproved of by society.
How is it? I know it's pretty reductive and binaristic, but I'm trying to construct a 'born this way' that doesn't require familiarity with the community in any way.