Humans like categories. We like things to be this or that. At some point in time some cultures decided that gender and biological sex were linked things and that there were just two of them. Now lots of cultures, particularly those indigenous to North America had words that accepted a little more nuance than that. Hawaiian has the word mahu*, to represent something in between.
I bring this up because whenever these discussions come up they tend to follow a couple of paths. Some people argue that being transgender is a new-fangled modern thing, despite the mountains of evidence that it is not. Similarly I see a lot of allies bringing up intersex folks. And intersex folks do, and it looks likely always have, existed. Where I get a little antsy is I personally am not cool limiting gender to biological determination, not the least because what's in my underwear is the business of a very limited set of people. So if I want folks in various places to stay out of my pants, then saying we'll stick with biology is not how we achieve this. Yes, sticking to a gender binary screws over more people than trans folks. And not even nature is using the binary is part of that argument, but the world I am fighting for is the one where the state of my sex organs is between me and a small group of people that I decide on. Not the TSA, not the DMV, not random person outside a public bathroom, so the argument I am making is that legislation to "undefine" trans people is harmful to us all. It isn't a trend we can get rid of, but I also will keep fighting to maintain the protections people, especially trans people need to stay safe.
And those of you arguing that you are keeping little old me safe from the trans people, nope. Nope, as we saw with recent hearings, y'all aren't trying to keep me safe from assault, you are protecting the assaulters. So my safety is intrinsically tied to the continued safety of trans folks. They have always been here, and they deserve continued and in fact increasing protections.
*This resource looks at quite a few Pacific Islander cultures and their words for folks along the gender identity and sexuality spectrum.
