I can see a difference between light-hearted speculation about an imaginary crab's gender and saying that non-gender-binary people don't exist. Can you think of any reason why one of these might be allowed, while the other might not be?
Not that that's necessarily wrong - I'm definitely sympathetic to the argument that the personal is political. But that argument would justify more technical communities taking political stances, not fewer.
If, however, some community members wish to do that, they open to others the option to speak their minds on the topic, including stating that such an action ought not to be taken because no such gender exists. Contrast this with how most "codes of conduct" are written: they would permit community members to speak in favor of such a decision but not against it, as doing so would be "hateful", "bigoted", or "exclusionary".