|Reddit thread here, if you're interested|
Sexism on the internet is a problem, and atheism/skepticism are predominant on the internet. So "sexism is a problem for atheism/skepticism," because the internet is a primary conduit of interaction for such people. The internet helps people around the world who feel isolated come into contact with others of their kind, yes...and it also creates the impression that those who take advantage of this opportunity speak for everyone in their minority group. When, for example, PZ Myers and Greta Christina disagree, who should we listen to? Who is the representative of all atheist-kind? Well, neither of them, obviously. Atheists and skeptics have no popes, no bishops, no chain of command, because-- this is important-- atheism and skepticism are not belief systems. Atheism is a lack of belief in gods, and skepticism is a tool, an epistemological approach. Nevertheless prominent atheists can influence the beliefs of others, and lacking churches and being in disparate locations, they often do so on the internet. People want to group together with others of their kind, and when they do they like to have authorities. People outside of groups like for those groups to have authorities to speak for them, to encapsulate what they're all about. It makes things easier, but in this case also distorts the picture grossly.
Bottom line: sexism is an internet problem. A world problem. The fact that atheists and skeptics on the internet are discussing its existence within their own ranks does not betray that atheism/skepticism "has a sexism problem." It means that there are vocal people who are concerned enough about this ubiquitous problem to address it, and that quite naturally leads to a widespread ongoing discussion. It's amazing how much easier it is to avoid having a "problem" with something when your community is either homogeneous, or the community is homogeneously in agreement on it (perhaps by silencing or just not listening to dissidents). Atheists/skeptics on the internet are willing and able to speak up, therefore they're the ones with the problem. Hmm, not buying it.