I'm getting pretty tired of some people tying themselves in knots trying to deny trans people their identities. It makes being a feminist trans woman really difficult when you have no allies at all when both sides of an issue have people so desperate to be right they will use any argument at all, no matter how much it contradicts themselves, to justify their position.
Let's break this down:
1) There is no such thing as inherent gender. Everyone's brain is born the same.
This is the one that I have the most sympathy with. People on the trans side of the debate often choose this as their hill to die on, pointing out that there are some miniscule apparent differences in the small number of trans women that have had their brains dissected that are more analogous to cis women than cis men.
Now personally I am not sold on this. If your identity is determined by your brain shape then that's just another physical difference for cis people to beat us over the head with. You say you're trans? Okay, climb in this MRI machine. Oh, look at that, you're not, now go away. Conversely, what if you're a butch lesbian or a femme gay man and you have the MRI and find out you're actually the opposite gender, do you become forced to transition?
Thankfully the biology of sex differences in the brain is hotly debated anyway, with new studies every other day claiming opposite results, so right now we don't have to worry about it. Personally I don't think either side should be hanging their bonnets on this one.
2) Sex is fixed and inviolate.
Sex is in itself a fairly arbitrary distinction. As I've noted before, across the billions of people on this planet every possible variation of the human body exists somewhere. Yes, most people are clearly one thing or the other, but we're not stamped out of a mold with male and female embossed on our behinds.
Intersex people of all kinds do exist, and they generally have to choose a gender, or have it chosen for them. The idea that the genitals you are born with determines the person that you are is both endorsed and rejected by these people at the same time, a staggeringly impressive piece of doublethink. "What would you know, you're just a gay man in a dress", they cry. Then "A woman can do anything a man can!" Okay, but which is it?
If we are not bound our sex then presenting and identifying with a certain set of people who usually have different genitals should be allowed, surely? If you want to use a different word for me just because of them then that doesn't really make a difference to me, as long as you treat me the same as you treat other people of my adopted gender. But you won't do that, because this debate only exists to justify your bigotry.
3) There is no such thing as gender.
This is the one that infuriates me the most, because in my opinion they're kind of right, but they've misconstrued what that means.
Now, sex is a thing, albeit not a clear binary. While it doesn't tie you to a personality type or a career path it does have clear and measurable effects on you. Testosterone makes you stronger and more easily aroused, for instance. Wombs grow children and have monthly pesky bleeding cycles that get in the way of life. To dismiss such things effect on your personality is absurd, but the net effect of all your experiences determine that personality, not just those things.
But people have lots of different experiences. Yes, most people's are similar in many ways and so most boys become men and most girls become women. But some don't. Some grow up never really identifying with either, but have an assumption of what they must be based on their genitals. Some grow up to identify with what most of the the opposite sex are doing. That thing that most of a particular sex is doing we call gender.
Gender is the physical stuff we see when we walk around and label people. We don't have a root around in people's underwear before deciding what gender someone is. Now what that means as far as how that changes how we treat someone differs enormously. Sometimes that can be a very bad thing, but that's true of any identity, from being old to being a goth. But it's absurd to say it doesn't matter at all. Maybe you mean it shouldn't, and I don't entirely disagree with that point of view, but the fact is it does.
There are so many expectations that come with our outward appearances that presenting as the wrong one is making life extremely hard for yourself. Maybe you like a challenge, but many people are tired of it, tired if having to justify their likes and dislikes, the way they sit, the way they walk, the things they like to do and see and experience that eventually they say look, this label just doesn't work across the board, if I take on this other one will you just stop bugging me about it?
If you really don't see gender, if you don't accept at all that it's a thing, then women's groups for discussion of anything but your biology should not exist. They exist because of shared experiences, and just occasionally those experiences are shared with someone with something different between their legs. You should try talking to them.