They are so open minded their brains have fallen out.
It is a challenge for anyone who counts themselves as being on the progressive side of politics to avoid falling foul of O'Sullivan's First Law, which states that "All organizations that are not actually right-wing will over time become left-wing". O'Sullivan gives a good explanation of why this is so, albeit couched in somewhat reactionary language here: web.archive.org/web/20100715191034/old.nationalreview.com/flashback/flashback-jos062603.asp
In this case, the general social movement that decided (rightly, I hope most would agree) that it's OK to be non-white, or a woman, or gay, and that those groups should have absolutely equal rights, seems to have also decided that whatever group of people came along next claiming to be oppressed must also be correct, and that anyone who pushed back against them must be as bigoted as those who would deny non-white people/woman/gays their rights. Saying "Oh your poor things, did those nasty right-wing newspapers say horrible things about you" became a substitute for thought, because obviously if the Telegraph or the Express doesn't like it, they are being horrible bigots. Hence why the majority of people with pronouns in their e-mail signature probably know zero trans people; they just want to be nice and not beastly to these poor people who are a bit different to themselves.
A further tragedy of this is that people who want to stand up and say no find that the only support they can get often comes from gleeful right-wingers. 90% of what JK Rowling believes would be laughed at by the majority of people who support her on Twitter, and Graham Linehan seems to have gone round the twist. I'm not sure what the right way to push back is, but it's something for the left to sort out because while we might like what Trump and Putin have to say about genderwang, we probably don't want the rest of the package that they're offering.