Because women should be hired for their skills. Not because they provide a good 'mix' to compliment the men on a team. To think otherwise is patronising, condescending; and as a result, sexist.
Edit: I'm genuinely shocked by the number of comments that contain a latent sexist pretext. Maybe I've been brought up in an overly liberal environment, but I find it pretty horrendous nevertheless.
That's because you're on a witch hunt. You put words in my mouth and then branded me a sexist based on that.
So, here's the thing. I actually consider myself a feminist. I've been outspoken in the past on things like believing that the gender gap in computer science and IT is primarily a consequence of social conditioning and that people should be evaluated and valued within an organization based on their qualifications and not based on their gender.
As such, I'm a bit uneasy with the premise of the article -- that work environments can and should be made more "girl friendly" (insofar as that goes beyond not tolerating overt sexism). But that premise came from the article, not from me. Personally, I'm conflicted there and could argue both sides. But, if we run with that premise, and try to answer the question in the title, I find it an impressive stretch to read sexism into saying, "I find mixed gender work environments more pleasant than single-gender environments."
So, if we have a group of people that, are broadly speaking, all the same type and we think adding someone with a different set of characteristics will lead to the group performing better as a whole...we shouldn't do that? We should instead higher the "best" individual regardless of how it impacts the group?
The goal is to get the most productivity from the team/group. Sometimes that means hiring for attitude over aptitude or ...oh yes, diversity - be it race, gender, degree, life experiences, etc.
When hiring anybody it's short-sighted to just look at their skills. If you want to build a great team you have to consider the effect the new employee will have on the existing team. Sometimes that mean passing up the person with the best raw skills and hiring based on other factors.
The whole is sometimes greater/less than the sum of its parts. The goal of a company is to maximize overall productivity, not to assemble a collection of the highest performing individuals. See Miami Heat vs. Dallas Mavericks in 2011 NBA championship.
Edit: I'm genuinely shocked by the number of comments that contain a latent sexist pretext. Maybe I've been brought up in an overly liberal environment, but I find it pretty horrendous nevertheless.