English evolved in a time period, predominatly ruled over by men. Hence, the default term is: "Mankind", "Man", and the default, when gender is uncertain, is to use "Man", "Him", or derivatives of such.
In Old English, the word "man" was gender-neutral, while the words for male and female were something like wer and wif. The compound word wifman, meaning "female human" is what evolved into the modern word "woman" (interestingly, the word wer survives most commonly in "werewolf", which as you can see literally means man-wolf, and distinctly male). Cognates of "man", such as the German Mensch in fact remain gender neutral.
The gender-neutral third-person singular pronoun is "they", with documented use at least as far back as Chaucer and Shakespeare.
And even ignoring that, "English was like this" is no reason for it to continue to be like that if the alternative is perfectly understandable. Languages change all the time for all kinds of reasons; we don't use the complex system of verb tenses from Old English, or hither, thither and wither, or yon and yonder, (etc.), so why should we feel obligated to use its pronouns? (side note: which were not the same as modern English pronouns; "you" used to be a second person plural object only, it was thee and thou singular and ye and you plural.)
But yes, "man" used to be gender neutral, and for most of the history of English "they" was the gender neutral third person singular.
Disclaimer: If you are prone to dismissing women's complaints of gender-related problems as the women being whiny, emotionally unstable girls who see sexism where there is none, this post is unlikely to interest you.
For your convenience, links to followup posts: Roko says; orthonormal says; Eliezer says; Yvain says; Wei_Dai says
As far as I can tell, I am the most active female poster on Less Wrong. (AnnaSalamon has higher karma than I, but she hasn't commented on anything for two months now.) There are not many of us. This is usually immaterial. Heck, sometimes people don't even notice in spite of my girly username, my self-introduction, and the fact that I'm now apparently the feminism police of Less Wrong.
My life is not about being a girl. In fact, I'm less preoccupied with feminism and women's special interest issues than most of the women I know, and some of the men. It's not my pet topic. I do not focus on feminist philosophy in school. I took an "Early Modern Women Philosophers" course because I needed the history credit, had room for a suitable class in a semester when one was offered, and heard the teacher was nice, and I was pretty bored. I wound up doing my midterm paper on Malebranche in that class because we'd covered him to give context to Mary Astell, and he was more interesting than she was. I didn't vote for Hilary Clinton in the primary. Given the choice, I have lots of things I'd rather be doing than ferreting out hidden or less-than-hidden sexism on one of my favorite websites.
Unfortunately, nobody else seems to want to do it either, and I'm not content to leave it undone. I suppose I could abandon the site and leave it even more masculine so the guys could all talk in their own language, unimpeded by stupid chicks being stupidly offended by completely unproblematic things like objectification and just plain jerkitude. I would almost certainly have vacated the site already if feminism were my pet issue, or if I were more easily offended. (In general, I'm very hard to offend. The fact that people here have succeeded in doing so anyway without even, apparently, going out of their way to do it should be a great big red flag that something's up.) If you're wondering why half of the potential audience of the site seems to be conspicuously not here, this may have something to do with it.
So can I get some help? Some lovely people have thrown in their support, but usually after I or, more rarely, someone else sounds the alarm, and usually without much persistence or apparent investment. There is still conspicuous karmic support for some comments that perpetuate the problems, which does nothing to disincentivize being piggish around here - some people seem to earnestly care about the problem, but this isn't enforced by the community at large, it's just a preexisting disposition (near as I can tell).
I would like help reducing the incidence of:
We could use more of the following:
Thank you for your attention and, hopefully, your assistance.