I like this analysis, too.
Whipping Girl is good. And also a good example of disagreement among trans people about gender—I think Serano is mistaken in several ways.
Thanks for the link! Never seen this site before.
I think this is a difficult epistemology to uphold, although I'm vaguely sympathetic, and you're certainly not the first or only person to say this. Scientists (and others) posit unobservable entities all the time. Electrons and black holes are classic examples. Those in favor are scientific realists; those who have issues with this are a subset of scientific anti-realists.
Frankly, I can't think of anyone who posits the existence of something that can't ever be detected. Presumably, if you're genuinely positing the existence of something, you've "detected" it!
I think you're using "observation" and "unobservable" in ways that many people aren't. We certainly don't need to observe anything to know the truth or falsity of some mathematical fact.