my experiences up to that point gave me no reason to believe to believe my presence would cause alarm
As a woman, seeing that is a very familiar thing, the experience of realising men don't know what it's like to be a woman, but think they know everything because they haven't stopped to consider that women could have their own perspectives and experiences outside of what men can know.
Women know an adult male, particularly an unknown one, is a potential danger, or may do something that is not in itself physically dangerous, but is upsetting (ogling, dominating, etc). This is partly instinctive. Part of keeping yourself safe is not letting on to the man that you identify him as a possible risk or are on your guard, because that may provoke him. Women not letting on to you if they are alarmed and even trying very hard to suppress it and cover it up does not mean they were not alarmed or uncomfortable.
There is a peace and relaxation and camaraderie and shared experience (I mean the full experience, not just some bloke patronising us at work or being wolf-whistled, but the shared understanding of having female bodies and female physical experiences) that comes with being with just women. (Of course women can be aggressive and dangerous and I'm not denying that, but it's rare and we know another woman isn't generally packing the strength a man is.) That is destroyed by a man being there. Some women might genuinely not mind that, but them not appearing to mind doesn't tell you anything.
I very much do mind. If I want to be with just women, that's what I want. On top of that I don't enjoy a male doing a stereotyped imitation of my own real experience, appropriating it and claiming to be what I am. It's rude, to say the very least, and in any other context (ethnicity, height, disability, age) that would be clearly understood.
This is why I completely stopped going to anything "women only" because I know such things attract TW and I couldn't face the loss of what I had sought out and not being able to have that peace. Not because I hate anyone or am "anti-trans" but because being a woman is real and being with just other women is sometimes what I want or need. So not only do the reactions of the women there not tell you anything much - you're also not even seeing the women who aren't there because you might be.