I see no reason in that for anyone to assume an entire internet forum to be exclusively one gender. Someone sees a set of interactions that they assume to be between a single gender, so they assume everyone is that gender?

It could be that it really is overwhelmingly male and that the women who do participate often downplay their gender, thereby making it look even more overwhelmingly male than it actually is, thus fostering the impression that it could be 100% male. It could also be that the lack of "social" elements, like avatars, makes it inherently hard to signal such details or ferret them out if you are curious about it.

(That is not a criticism. It is an observation. I like the way profiles are handled here.)