I never intended to imply that this was a conscious assumption, but I do believe the assumption is there. Language matters and shapes our thinking, and there is no reason to think that "everyone does it" doesn't mean it's sexist. Even where is a cultural norm, it sets male up as default and women up as weird variations from that norm.

It has been shown in German and Russian that the gender of inanimate nouns changes how people respond to them. Why do you think that wouldn't be true of referring to everyone as male?