Thanks to everyone who has replied. You have helped me process this experience.
A couple of things...
I had no interest in outing her, either privately or publicly. That would have been the farthest thing from my mind. Nor did I have any interest in some sort of trans-related chit chat or Q&A.
My desire to maintain a poker face was really (I think) about two things: First, not wanting to puncture her dignity by reacting in any way, and second, not wanting to let on that I know a whole lot more about the subject of gender identity than I would generally care to admit publicly. When faced with trans-related subjects or conversations, I completely go into self-preservation mode, lest I give away my little -- er, big -- secret. I'm very good at reacting and speaking like any (non-trans) LGBTQ ally might. But it's all an act, intended to carefully hide where I really am with the subject. (I know this is bad. I hate it. But I'm not in a position to come out, and I've been keeping this secret for so long that it's basically like muscle memory now.)
Next, I DID feel like my reaction was at least in part transphobic, and that disturbed me. Unlike some others, I was raised in a very open-minded household and had my first contact with the LGBT community (though it wasn't called that back then) before I can even remember. My parents were both in theater...need I say more? When I told them (about 30 years ago) that I was a crossdresser, they more or less shrugged and said, "We love you."
But still I have that gut reaction -- noticing, becoming self-conscious, feeling awkward -- even when I recognize that the other person has simply gone where I hope to go someday. It seems like I am hard-wired to react to anything outside the traditional social norms, even if I support and accept and understand what I'm seeing. Why couldn't I just react to her -- or NOT react -- like I would have with either of the other two women who flanked her at the check-in desk that day? I don't know the answer to that question, but I think such xenophobia may very well be hard-wired into our species. It hurts to find it within my own wiring. That feels like an important subject for the trans community to wrap its head and heart around. (Of course, I know this is not new.)
Finally, I think my experience is very instructive in how I would have to approach transition. What I find startling in others becomes a huge hurdle for me in transition, knowing that going about my everyday life would be causing these types of reactions in others at basically every turn. I shouldn't care what others think, but I do. I shouldn't. But I do.
I could never be passable, but I expect that people would likely try to avoid puncturing my dignity by suppressing their reactions like I did. After a while, this social mechanism could lead me to think that maybe I WAS passable, which would be mistaken. It's perhaps a harmless mistake, but with enough time it could lead to a level of self-delusion which would be ripe for a hard fall.
And I think this mechanism is at work a lot in our human interactions. I can think of other things that I might notice about someone else which would make me want to hide the fact that I had noticed in order to avoid treating them any differently or puncturing their dignity -- all while my brain is racing a mile a minute trying to figure out how to do that.
Sometimes I think I should go find a cabin in the mountains somewhere, where I wouldn't have to interact with people at all! It would be so much simpler...
Lora