It is sad that some people dispute the existence of sexism in tech, but that does not make it controversial in the sense that it is contentious, because it isn't. The facts are simple: 1) technology is a big source of power in our society, and 2) women are underrepresented in tech. Whenever a well defined group of people is absent from (or underrepresented in) a source of power, discrimination is taking place, period. Sexism, like racism, is a loaded word, and people often believe that malice is required for it to exist, but that is not the case. Whether sexism begins in kindergarten or in mens-club startups is, at this point, secondary. It is there and we
must address it.
In the case of GitHub specifically, the issue is even clearer. This is a company that for years had a blog with little else than their drinking escapades. Perhaps that's changed, but the burden of proof is on them.
The only thing that is up for dispute is whether this particular case is an example of sexism in tech or not. We can't be 100% sure, but at this time it's safer to assume that the weaker party is right, if only because the stronger party can handle a quick-to-judge public much more easily than the weaker party. I'm not saying it's boycott time, but it's best to at least open the discussion with that assumption in mind. Again, the burden of proof is on GitHub.