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?
The original poster is saying "somehow in my head HN posters appear as male". That doesn't really mean the poster is assuming that HN is actually 100% male. It's a description of his or her imaginative process, which is a matter of great psychological and sociological complexity.