I wouldn’t particularly spend time trying to convince people like that that my existence matters and would hardly consider them comrades. However, if you do want to, then I guess the first step would be to point out that rather than a sign of western societal decay, trans identities supercede western society, and, in fact, cisheteronormativity is a western construct created to allow the capitalist accrual of wealth and power by a very select few (cis het white men). The destruction of more nuanced understanding of gender (and the elimination of third and other genders) is a colonial effort. The main problem with people who don’t “believe in trans identities” is that they clearly also don’t believe in reading about non-european cultures or history. Another thing to mention would be the the provision of gender affirmation care in actually socialist countries. For instance, before the fall of the anti-fascist barricade in Berlin, East Germany provided gender affirmation surgery. Cuba also currently provides gender affirmation medical care. You might also mention that one of the things the Nazis were very keen on was the destruction of the libraries of the Institut für Sexualwissenschaft, where they eradicated a lot of research into queer identities and sexuality before slapping pink badges on trans people and throwing them into concentration camps alongside the Jews and the commies. You might then question why a supposed comrade would be so happy to side with fascists on their head-empty opinions of queer identity.

A bullet on their heads.