You don't, but some people do. Not necessarily just on this issue either -there are interesting discussions to be had about race in a similar context.
https://medium.com/p/c5c64faa2421
I wrote such a story about John Boyega when he said he would exclusively date black women. That was celebrated and condemned in equal measure back at the time and it posits some interesting questions at the intersection of personal choice and group-think.
General statements about groups is what identity politics is founded on - it's one of the reasons I don't engage in it. Individual trans people are potentially alien, bizarre and unreasonable but has nothing to do with being trans and everything to do with them being alien, bizarre and unreasonable as individuals. That is also true of other people who aren't trans and everyone else on the planet... the issue we seem to have across society is we keep giving the microphone to the alien, bizarre and unreasonable people.
There's nothing inherently wrong with having different definitions or even different worldview, the important thing is we can talk about them. If you're interested in my views then here are a couple of stories to get you going and perhaps the algorithm will do the rest.
https://medium.com/p/1479823395d4 - that's just a general overview of the mess I think we're making of the argument.
https://medium.com/p/2af13254adcb - that's some thoughts about Nicola Sturgeon (written before she imploded)
https://medium.com/p/86d6cb79aa11 - and that's a call for trans-people to be careful about who they let advocate for their community.
I have enjoyed this back and forth. It's good to have respectful chat on matters of disagreement, consider yourself followed. We may not always agree - but feel free to hold me accountable for my views in my comments section and expect me to pop up and politely dissent when I think you've over-reached on the identity issues.
As a heads up, you aren't going to like my views on the incompatibility of having trans and non-binary in your bio. ;o)