StopThePlanet So very sorry that happened to you. Your reaction was neither unkind nor over the top nor unjustified nor transphobic.
I would have got an almighty fright too if a naked bloke had suddenly loomed behind me in the women's changing room. Depending on how frightened, I would have screamed and/or asked that pervert what the fuck he thought he was doing coming anywhere near me without his clothes on.
Now it may not be defined as flashing as per the gym's new rules, but it is intimidating, harassing behaviour nonetheless to react to someone who got a fright in the way that he did.
I would be interested to know what your state's laws say about this situation. The law may well consider it sexual harassment whatever the gym thinks. And I would think it worthwhile to consult a lawyer and eventually to contact the press about such an incident.
Right now the gym has the upper hand because you feel guilty for your perfectly natural and justified reaction (one could even argue that this is a perfectly logical trauma reaction for an assault victim), and because you want your money back.
But if the gym renewed your membership and did not inform you of the new terms, they may be in breach of contract. I do believe even in the US any changes to a contract (which the gym's T&Cs form a part of) need to be communicated to the other party.
These new conditions of the single-sex facility no longer being single-sex would presumably have prevented you from entering in this contract in the first place and would definitely have led to you not renewing had you known. It's a material change to the service they provide to you as a customer after all. So, I would at the very least get a lawyer to send the gym a letter arguing that. If you can afford it.
In any case, I have never come across a naked woman in a changing room behaving like this - walking up behind a complete stranger and then berating them for getting a fright, actually have never come across a naked woman walking up behind a complete stranger full stop.
What possible reason would there be not to call across from the other side of the room to get someone's attention for instance or for wrapping a towel around yourself first.
We always get dressed swiftly and full nudity is kept to an absolute minimum amount of time. No open staring, creeping up on others, stupid remarks or whatnot. Mostly not even any eye contact if I'm honest.
So this pervert - who from his behaviour of letting it all hang out in the women's changing room is either a straight up predator or an AGP with a relevant fetish or both - wanted to know what you were washing in the sink. Is that normal behaviour? No it isn't. It's fucking creepy and wildly inappropriate.
I hope you can find a different gym soon, one that is much less hostile to women.