I think it is an automatic reaction isn’t it? It’s a lifetime of just ‘knowing’ what sex people are. I recently met a transman - friend of a friend. Lovely person - I really liked him and we got on well.
I’d seen pics of him on social media before we met and he looks like a man. I had never had any trouble referring to him as a man, using Male pronouns etc. But when I actually met him I had SUCH trouble not saying she and even now I’m finding it tricky - though I would never want to upset him and I would try hard not to.
Because he is very slight - probably 5ft 2ish with delicate features my brain sees him and thinks woman. He was just wearing jeans and a t-shirt (same as I was) and consequently I found that in real life, he looks like a pretty woman with short hair. It’s like a constant clash in your brain. At least that’s what I thought.