I think one of the main reasons why trans rights are such a common litmus test is because it demonstrates how easily someone is willing to live-and-let-live, and to have space for others.
The reality is literally the other way around of
reactionary talking point
"forcing their way of life onto us".
If someone says "I am a X" and a dweller responds with "no, you are not a X, and here's why I am so certain about that as a third party", that is an act of forcing their worldview/lifestyle onto other people.