Are there any women-only places in the world?

The Riyadh Women University campus in Riyadh, Saudi Arabia could qualify as an answer to your question. Its new campus can accommodate 40,000 (female-only) students and 12,000 (female-only) employees. There are only women because of the Saudi laws of sex segregation. Inaugurated in June 2011, it spreads over 8 million square meters (2,800 km²) with 800 buildings and ten gates between Riyadh and the university campus, the total built up area being around 3 million square metres. In addition to the university buildings (13 faculties, a library of 6 million volumes, administration, research centers, laboratories) and housing for university staff and students, it features mosques, schools and kindergarten for children, as well as sport and recreation facilities. It also includes a nursery for young mothers and as well as a training hospital with 700 beds. The large campus and community is served by an 11.5 km automated metro. It is one of the ten largest universities in the world and the largest university for women.

The expansion of the campus was done in a record time of two years supported by 75,000 construction workers and latest building machinery and methodologies, at a cost of 11.5 billion US dollars.

You can find videos on YouTube giving you a look at the construction works or at a small-scale reproduction and at the inside of a few buildings now that their construction has been completed.

Both Buddhist and Christian traditions have women-only convents or monasteries, also known as nunneries. Some of these are nowadays partially open to visitors, such as the Chi Lin Nunnery in Hong Kong or the Convent of Las Descalzas Reales in Madrid. I don't know whether these are the most authentic or noteworthy, though.

An entirely different possibility: in some poorer countries like Armenia, a large percentage of the men seek work abroad while the women stay at home, which can result in villages being almost entirely populated by women.

The place doesn't have to be particularly large, but there has to be a cause of it being mostly female.

At the (gorgeous) Pärnu beach in Estonia, there's a somewhat large section for women.

^{Photo by me, July 2012}

The sign doesn't explicitly state it, but apparently men should stay out, sadly. :)