I mean it’s really all just a back and forth. In my experience, the “trans women and cis women are different because…” conversation comes up when the ultra-SJW crowds make assertions that cis women are the exact same as trans women, and acknowledging anything else is offensive. I mean Twitter is a wild breeding ground for facetious statements that get taken seriously and snowball into a biology lesson all because someone made a tweet like “if you don’t date trans women you don’t actually like women” and then a group of cis women feels attacked by it and it goes from there.
Sex and gender are not tied at all, that’s very true. Gender only exists because of dualities.. but not to be rude, you are entirely ignoring the group of people who are not transphobic and are trans-allies, but still know that there is a difference between trans women and cis women – and that is their actual sex. They have the same gender, but biologically, and factually, they aren’t the same – that is one has a female reproductive system or most of it as well as female hormone dominant and the other doesn’t have a female reproductive system and wasn’t female hormone dominant until beginning HRT.