Is being "trans" a matter of science (female personality in male body, or vice versa), or is it a matter of political choice?
The very idea that there could be such a thing as a "female personality" is not scientific, it is a matter of political choice.
The only scientific fact, if one can call it that, is that some people have body dysmorphia.
No one in their right mind would suggest that people with a certain kind of delusion in their head should be treated as if their delusions were reality.
If a woman with anorexia thinks she's actually obese, exactly zero people treat her like an obese person. People will sit on the place next to her in public transport, even though in her delusions, this place is filled by her body. If she goes horseriding, no one will ban her from riding the small pony as they would an obese person. And if she goes to see a doctor, she will be told to eat more, not less.
Likewise, a man who thinks his right arm doesn't "belong" to his body. He will be told to leave the disabled facilities to disabled people. People will not offer to fetch things for him.
It is only with transnonsense that we are asked to treat people who suffer from a mental health problem as if their delusions were real.
No one on mumsnet has ever said that we should not acknowledge the mental health problems of people with delusions about their biological sex.
I am sure it feels terrible - much the same way as having anorexia or believing that your arm doesn't belong to you feels terrible.
What we oppose is being forced to play along with their delusions to the point that we are made to risk our very lives for the sake of playing along.