Can you say a little more about what a "protected" feature is in this context?

That is, suppose I wish to form a community in which Xes are unwelcome, and I want to predict whether you will dislike it if I announce that wish.

I'm pretty clear that you will dislike it if X in {woman, Caucasian, queer, attracted-to-men, attracted-to-women} and not (necessarily) if X in {unwilling-to-pay, non-expert-in-$FOO}.

What about X in {adult, American, non-recovering alcoholic, hearing, sighted}?

More generally: is there a rule I could apply to predict the result for given X, supposing I knew the relevant demographic facts about Alicorn?

I do not like it when people announce that they wish to form communities I would be unwelcome in because of a "protected" feature (sex/sexuality/race/whatever).

I would recommend that you make an attempt to free yourself of this dislike. While some 'protected features' have few real consequences (skin colour won't make you out of place anywhere except possibly a tanning salon), many do, gender definitely being one of them. Sometimes it will be easy to ignore or work around those consequences, and sometimes it will not.

If brazil84 does sincerely find that his ability for rational discussion is hampered by female pheromones (metonymy), and you do happen to emit female pheromones, you certainly shouldn't be blamed for that, but neither should he - it is a weakness of him, but one that falls well within the demands of a modern and open society. While you are absolutely required to behave in a civil manner even in the presence of romantic attraction, it is acceptable if it causes you to perform less than optimally.

As long as he (a) recognises that in a modern and open society, he will eventually need to learn to think and behave rationally even when surrounded by women, and (b) does not by his actions prevent women from joining a rationality club, I do not think he is going out of bounds when he wishes to establish a group that caters to this need of his.

I hate that I cannot come up with a better, less loaded example (I fall into a pretty damn privileged demographic), but: I would understand if I needed to join a rape victim support group and ran into one that wouldn't let me in because the women members were uncomfortable to talk about these experiences around a 191cm / 90 kg bearded deep-voiced man. Of course, I would find it unacceptable if that were the only support group in my area and it chose to leave me completely in the cold rather than create some discomfort to other members.

Using an arbitrary and subjective "pointlessness" exception, you can derive any conclusions you like. Just apply the general principle consistently as long as you like the conclusions, and otherwise proclaim that the "pointlessness" exception applies. And voila, you can bask in the glow of your high principles, which just happen to imply conclusions to your complete liking. (Of course, the general principle would produce absurd and impractical results if really applied consistently, so someone who subscribes to it has to operate with some such unprincipled exceptions.)

The distinction between "protected" and "non-protected" characteristics is of course similarly arbitrary and ultimately serves a similar purpose, though you don't have personal control over that one, as the power of defining it is a prerogative of the state.

That said, I don't mean to point a finger specifically at you by pointing this out. This mode of thinking is all-pervasive in modern society, and nobody is immune to it completely. But on a forum dedicated to exposing biases and fallacies, it should be pointed out.

Could you characterize what you mean by a "protected" feature?