Perhaps women are less underrepresented on Less Wrong than may at first appear, and men are more likely to comment for some reason. Or perhaps women are less likely to choose visibly feminine usernames.

I think it's pretty frequent on the Internet, especially among those that have been around since the early-to-mid 1990s, to assume that everybody online is male by default until proven otherwise.

Being one of those, my first guess would be that the LW audience is > 90% male, but I'd love to see the results of an anonymous survey...