they're guessing based on my location ("the Internet" or the specific site), which amounts to careless, casual stereotyping and rankles horribly

My prior probability that someone is male is about 50%, knowing that they read Less Wrong amounts to about +10 dB of evidence that they are male (as of the last survey), so the posterior probability that someone is male given that they read Less Wrong is about 90%. How does Bayesian updating amount to careless, casual stereotyping?