Thank you for your thoughtful response Squircle. A lot of what you discuss has been on my mind since Thursday. A lot of cis attitude seems to boil down to the fact that they see someone who is trans as being intrinsically the gender they were assigned as not the one they identify as, regardless of how well they pass, how long they have been on HRT, or whether they are pre/non/post op.
As trans people we know we have identified as our preferred gender for a lot, lot longer than we have been on hormones or seeing a counsellor (usually our whole life). As most of us have experienced though, being on HRT does change aspects of how we approach life and people. Being trans usually means being forced and socialised into gender roles that are alien and anathema to us, but that doesn't mean we don't pick a lot of it up along the way and bring it with us when we do come to transition; HRT may take the edge off that but we have still lost many years of sociological and biological experiences (good and bad) that can make it a challenge to fully relate with and to our cis siblings.
I understand that for the majority of cis and genetic women they have had the threat of male sexual predatory behaviour hanging over them all their life, some of them have been raped and/or treated in a sexual degrading way (often by family members) and often more than once, most if not all have been the target of harassment the moment they walk out the door....they feel threatened and oppressed by men at home and outside home. So a private space for women to just be themself is a great thing. It was largely why I sought out this gym.
When it comes to transwomen being in that private space, no matter how well they pass or how well they behave, genetic women are probably in many cases thinking "that is a man" and those fears about men are immediately there. I have a lot of sympathy for that fear, and having been on the receiving end of some scary male behaviour towards me I also have a lot of empathy. But I don't have any sympathy for the underpinning belief that we are somehow still essentially male because our birth biology doesn't match our brains, that we are somehow at best a second class of woman who needs a Proof of Vadge badge to access cis gender privileged areas.
I must admit that in approaching this gym I had hoped I would pass well enough for it not to be an issue, apparently for a few women they doubted my "authenticity" the moment they saw me, spoke to management and then some obscure "needs a vagina to be a woman" policy was brought to bear (and thanks for your comment ftmmax, that has been going through my mind about how welcome pre/non op trans guys would be!). I say obscure because I have found no evidence of it and it certainly isn't in their contract.
At the heart of the matter is lack of awareness, understanding and compassion of and for the trans experience. The second they saw me as trans they essentially defined me as "really a man" and we all know that's exactly how a lot of people think. They then told me they needed proof I was "really a woman", a deeply humiliating and upsetting experience.
So yeah, what to do? I have a few ideas but they will have to wait for another post.