I've often found the opposite to be true. I think that most people who don't know the sex of a forum member do have a preconceived idea: that the forum member is male. Statistically, that's a good bet, but not a sure bet. If they later learn that he's male, they don't change their style of conversation. If they instead learn that she's female, they may change how they converse with her. That's a generalization, and not always the case, nor necessarily good or bad, just something I've noticed.
Another good reason that people should decide for themselves what personal information to give out.
I was thinking, coming from a community that emphasises the equality of women and men, that being on this forum and speaking to everyone as a human being, uninfluenced by gender, is for me sometimes more like that than real life in the outside world.