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.
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.
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."
This part reads like veiled sexism to me. Sure, there are disclaimers and "I-don't-knows" and "might-be's" thrown about but in the absence of real data I don't think there are good reasons to bring this up other than to reinforce sexist stereotypes.
Do you really consider it far fetched that there could exist a team that worked better when they were all the same gender? I'm not asking if you'd like such a team (I wouldn't), but it's fairly obvious to me with the variety of people that exist in the world that not only could such a team exist, but one almost certainly does. Have you not met a half-dozen socially awkward geeks (male or female) in your life that struggled with professional interaction with the opposite of sex?
An important mechanism of earnest debate is the ability to differentiate between ideals and observed reality. I, for instance, find dogmatic fundamentalism unsavory, but it obviously has the ability to wield both a cohesive social force and give rise to political power. It's intellectually dishonest to assume that things we don't like are unable to produce advantageous results.