I have an idea. I spend far too much time on twitter and have tried to engage in good faith with many transgender women. Just my own thoughts, not a working scientific hypothesis mind you. But most of them are in the 30-45 age bracket and gamers. I suspect that these were young dudes who were chatting online while gaming. They toyed with the idea of presenting as female in chat boxes. They noticed how much more attention they got from the other male gamers, they were the centre of attention. So they started presenting more and more as female and enjoyed all the flirting and attention. As they started socialising more they struggled with keeping their male identity, flirting was natural but these were not homosexual males, they were used to presenting as female to get attention, they didn't associate attracting male attention with also being male.
To Coburg's attempt at obfuscation on biology: no, mammalian biology isn't really that complicated. Trying to dig into to the minutiae of any subject will give you constantly bifurcating results. Reductionism is often unproductive. Blue is a colour. But it's also a spectrum, with no clear boundary to purple at one end or green at the other. But sexually reproducing mammals have 2 completely distinct gametes. There is not a spectrum here. This is a simple binary. Those who we socially refer to as female produce one type, those we refer to as male produce the other. The miniscule proportion that have a condition that renders their sexual organs ambiguous are generally referred to as intersex and do not produce a 3rd gamete. And certainly not a spectrum of gametes. And to be very clear, intersex people have nothing whatever to do with transgender people. These are completely different categories. There is no evidence transgenderism has anything to do with biology.
And yet there are those we'd refer to as male or female who don't produce any gametes. Reductionism is useful for large scale classification, less helpful for the individual who is reduced out of existence. To be clear, I'm not saying male and female don't exist as useful classifications. It's a system that works for most humans, quite a lot of mammals, a significant number of vertebrates, but then kinda gets murky in the rest of the living world. Still, the overwhelming majority of people fall nicely into these categories, even if they have to squeeze themselves in there.
Also, there certainly are a spectrum of gametes. Non disjunctions, translocations etc. And that's just for chromosome number/size. In fact, the fact that a spectrum of variation exists in both the production, and combination of gametes is the reason for the (relatively recent, but incredibly significant) success of sex as a biological process at all.
Are these significant variations from the norm common? No. Do they often result in a lack of viability? Yes. But not always. And we're not talking about the norm here. That's kind of the point. The norm is a collection of correlated phenotypes we refer to as male or female, which approximates a binary separation. Just because most people aren't the confusing individuals on the edge of those limitations, doesn't mean those people don't exist.
And yes, intersex is different to transgender, as a classification. Mainly because being intersex is easier to quantify and measure. But looking at intersex people, the guevedoces were an example I mentioned, you can look at how easily psychology doesn't match up with physiology. When you realise how many genetic pathways there are to variation in the standard male/female distinction, it is a relatively small cognitive and emotional leap to empathise with those who feel one way and present another.
This whole discussion is exactly analogous to the discussions of homosexuality through the 60's and 70's. Firstly, the denial that its anything other than people with vested interests. Secondly, claiming they don't exist at all. Then the fervent search for the 'gay gene', before realising that biology is *smile* complicated. And that the millions of interconnected neural and hormonal pathways that make someone who and what they are can lead to all sorts of possibilities.