i answered that question at length here.
in short, it’s transmisogynist, obscures the dangerous legacy of colonialism, and is exclusionary. you can’t take away something that someone has never had. if the most at-risk groups within nonbinary communities (PoC, camab trans people, etc) cannot safely access a vital resource that should be available to us all while more socially accepted groups (white, cafab trans people, etc) can, then the nature of this political “gain” is drenched in another layer of oppression that ultimately strengthens and reinforces colonialism, transmisogyny, and capitalism. read all of these. also, support trans women, camab trans people, and oppose colonialism if you wanna improve the social standing of trans people. elevating (white) cafab trans folks’ social capital and power at the expense of people oppressed under transmisogyny will do exactly the opposite, which is how “legally recognizing gender” will function if actualized during this time. as long as cafab trans people have the power to extract value from camab trans people, things of this nature will be a dangerous idea for trans people. basically, Why Do Y’all Want To Oppress Trans Women and People of Color So Much.
racism didnt end with slavery and sexism didnt end when woman got the right to vote homophobia wont end with equal marriage and transphobia wont end because of bathroom bills when the hell are you ignorant fucks going to realize that the state of the law does not effect the state of society as a whole