Most mainstream feminist subreddits and other feminist and social justice groups see gender critical posts as outrageously sexist.

Men also think feminism is a sexist matriarchal movement advocating male genocide. People being stupid and ignorant doesn't really say much beyond the fact that they're stupid and ignorant. Liberal feminism and trans activism has continuously failed to argue their points without advocating patriarchal ideology. You can see this play out numerous times on this very sub. Just saying that a group is "sexist" because it gets men's dicks angry doesn't mean a thing. Can you actually prove it? There has yet to be a single trans activist that's been up to the task and capable of forming arguments beyond "Transwomen are women because I say so". The reasons why trans activism and liberal feminism go against female rights have been continuously explained by GC, and trans activism has never been able to even attempt to disprove any of it.

I believe sex matters in terms of biological functions, for instance abortion rights and menstrual pad access.

So...you acknowledge the reality of women's biology...but women themselves are a figment of imagination? How do you work with this level of cognitive dissonance? It's like saying "I acknowledge human biology, but I don't think humans are real"

I think restrooms and changing rooms should be gender neutral.

And the threat of male violence, assault and harassment that they exist in the first place to protect women from are just...what? Going to be whisked away if women stop being so exclusionary towards men?

Wait, are you the same person who's continuously lied about the reality of violence and harassment that women face at the hands of men even after being given copious evidence proving you wrong?

fetuses can't change their gender.

Why?

I said although people AFAB are mostly treated like men, they will still be subject to issues specific fo people AFAB.

And why do you think that is? Who do you think made it so that "AFAB" people don't have penises and need sanitary pads for their periods? The TERFs? The patriarchy? Cis women?