Actually, as another poster said, I saw your name and a few personal details in your profile and did enough research, including seeing a photo that is probably you, to be 95% sure you are male. I suppose it is extra-annoying because the person was wrong in this case, but I would still find that post lacking if you do prefer to be called "she."
I don't have a problem with your response in general, or ideas like "male should not be the default gender". I often try to avoid using gendered pronouns, for reasons like this. If the original person had said something like that, I probably wouldn't have replied. I do have a problem with someone casually questioning the use of a word, without demonstrating any consideration of whether it is actually correct, whether an effort was made to make sure it was correct, etc. For all we knew, the person who used "he" personally knows you, and mindfully chose to use a male pronoun. That uninformed, knee-jerk reply ("He?") is as useless and obnoxious as having a bot post "PUBLIC SERVICE ANNOUNCEMENT: REMEMBER THAT NOT ALL HACKERS ARE MALE. WE WELCOME WOMEN!" every time someone uses the word "he."
Really, to go full-on PC for the benefit of trans people, etc., I guess we can't use gendered pronouns even if we do know someone, unless we have been explicitly (and recently) told which pronoun they prefer.