It's really simple.
Some people are different - and they cannot prove this difference, but it affects how they experience the world.
They may be gay, bi, ace, pan or trans. Or have a disability you don't understand because you can't see it and haven't experienced it.
So, listen to them.
if you don't understand trans people, then go and read their own words.
The key is this: do you accept someone's difference, or do you need it proved to you? If so, why?
Trans people are, quite simply, who they say they are. Accepting that you are trans or non-binary is usually a long process which often involves adopting other labels along the way.
If you are sincere, then thank you. But really, you don't have to understand people who are different, just accept them. I don't know what it's like to fancy a woman, or sleep with one - but I'd never question whether gay women exist because I can see that they do. Difference is normal, and I have no interest at all in asking: but why? Why do you exist?
Please ask yourself: would you ask all these questions of a gay person if you yourself are straight? I doubt it. You'd recognise how rude it would be.
The concept of "non-genuine" trans women is a transphobic myth. Trans women have jobs, lives and responsibilities - which they carry out while trans. That's it, that's what being a trans woman is.
A few cis men seem to have tried to gain access to female prisons, but for the most part there is absolutely zero advantage gained pretending to be a trans woman. 'Safe spaces' are a myth. The immense social pressure, difficulties negotiating medical transition, physical exhaustion and danger experienced by all out trans women should be obvious. Trans women are targeted for rape, harassment and violence.
Step outside the GC bubble and understand that trans people are normal humans. Yes, they might look different - so what? Repulsion at a visual difference isn't proof that someone shouldn't exist.
Being trans is normal.
Sexuality isn't gender.
Difference doesn't require an origin story.
Trans people are gay, straight, ace, bi, pan and more - just as straight people are.
Being trans is not a costume, it's who someone is. Plenty of trans people don't dress according to gender stereotypes - but if they dont, transphobic people ridicule them for not looking masculine or feminine enough. This is the point of declaring pronouns - my son often doesn't look trad masculine, but if he's misgendered at college, someone will say "they use he/him" and the reply is always a variant of "okay, cool".
Which, honestly, is great.
it's so difficult to live as an out trans person, that most would choose not to be trans. It makes life a lot harder. No one would be trans unless it meant everything.
Finally: do you remember when parents bombarded CBeebies with complaints because a presenter had limb difference? It was claimed that by being on-screen acting like it was normal to be visibly disabled she was traumatising children. That was repulsion at difference. And, if course, this was exactly why it was so important she was onscreen: to teach children that difference was normal, and didn't need to be interrogated.
So much of transphobia is anger at visual difference. Getting past that, is the first part of the battle. Interrogate why you can't just accept this difference, but need it explained. Why can't you just believe trans people exist, and leave it at that? They're a tiny, oppressed minority, not ever likely to affect your life.
You seem like a good person, and I'm really sure if you met someone trans you'd just chat to them without needing to know anything about their bodies or sex life. You'd talk to them like they were normal, judging them on whether they were friendly or kind - just like anyone else.
Read their own words please. And, thanks.