Now, it is obvious that there are fewer women writing it and reading it, just like it is obvious that Wikipedia is primarily populated by weird, militant goofballs. If you read their Science 2.0 entry, for example, it claims that Science 2.0 came into existence in 2008, as part of the Open Science movement, and that this site does not even exist and all people who have at various times tried to correct the errors have been scolded and overturned by the marketing person who monitors that page. There is a good reason why we make goat noises at people who use Wikipedia as a science source - it's too easy to hijack.
But sexist? No, they are not sexist. There is no way to be sexist, no one knows anyone's gender. The culture may be all assholes, and maybe women like to fight with assholes on the Internet less than men, but that is not sexism, as long as they are assholes to everyone. And they are. Pew Research also found that women are not only less likely to edit but also use the site less than men, just not as much less as Wikimedia found when it asked its own community.
IT academics have calibrated the data from the original finding regarding genders of Wikipedia editors with the Pew results and determined the situation is slightly better than thought. There is a reason they care; one of the authors of the PLoS One paper is on the advisory board of the Wikimedia Foundation and likely does not see the issue the way outsiders do.
Their new estimation is that the proportion of female US adult editors was 22.7% instead of the 17.8% originally stated, and that the total proportion of female editors was 16.1% versus the original 12.7%.
Credit and link: doi:10.1371/journal.pone.0065782
So if Wikipedia is sexist, American Wikipedians are less sexist than Europeans. Take that, France!
Citation: Hill BM, Shaw A (2013) The Wikipedia Gender Gap Revisited: Characterizing Survey Response Bias with Propensity Score Estimation. PLoS ONE 8(6): e65782.doi:10.1371/journal.pone.0065782