The problem in calling just one male, 'she' is it creates a lie and it blurs boundaries. Boundaries that serve women (and children) in terms of understanding who someone is.
For example, a and b talk about their friend Alex and how they are going to stay with 'her' in a shared dorm. C is invited too and says yes on the basis of her.
Alex is very very nice but C is Muslim and this is a problem but C has had the ability to make informed decisions taken away because the boundaries were blurred. This is something of an issue when they arrive at their destination at 11.30pm and meet Alex there.
There are many similar situations where even though someone is very very nice - the point is that there is an implicit giving of information when you use the simple word her or she that would potentially make women make different decisions, if they knew it was a man. Because it is still important to them.
They don't want to impinge on the life choices of the transwoman, but they do want to make their own choices based on the knowledge of the situations they may put themselves in.
A woman who would never allow herself to be alone in certain situations, should be afforded this ability to self exclude.
Language absolutely matters because it prevents confusion or misunderstanding or really rather embarrassing situations for all concerned.
And from my point of view the use of language is particularly important and absolutely matters to many siblings / other family members in terms of their own well being and identity.
It erases my history and lived experience if I were to refer to my brother as 'she' or my sister. Identity is relational and other do try and find common ground based on shared experiences. I don't want to have to put up a whole pretence nor lengthy explanation - especially to complete strangers. 'He' is of relevance to my life and my experience and the words use matters to me.
You may not share that opinion but it matters to others.