From the Wikipedia entry on Mount Athos, home to a number of Eastern Orthodox monasteries and known by Greeks as the “Holy Mountain”:
There is a prohibition on entry for women… to make living in celibacy easier for those who have chosen to do so. Monks feel that the presence of women alters the social dynamics of the community and therefore slows their path towards spiritual enlightenment.
In the 14th century, Tsar Stefan Uroš IV Dušan brought his wife, Helena of Bulgaria, to Mount Athos to protect her from the plague, but she did not touch the ground during her entire visit, as she was carried in the hand carriage all the time.
French writer Maryse Choisy entered Mount Athos in the 1920s disguised as a sailor, and later wrote about her escapade in Un mois chez les hommes (“A Month With Men”).
There was an incident in the 1930s regarding Aliki Diplarakou, the first Greek beauty pageant contestant to win the Miss Europe title, who shocked the world when she dressed up as a man and sneaked into Mount Athos. Her escapade was discussed in the 13 July 1953, Time magazine article entitled “The Climax of Sin”.
In 1953, Cora Miller, an American Fulbright Program teacher from Athens, Ohio, landed briefly along with two other women, stirring up a controversy among the local monks.
A 2003 resolution of the European Parliament requested lifting the ban for violating “the universally recognised principle of gender equality”.
On 26 May 2008, five Moldovans illegally entered Greece by way of Turkey, ending up on Athos; four of the migrants were women. The monks forgave them for trespassing and informed them that the area was forbidden to females.
Men are banned from the mystical island of Themyscira, too, but being as it is a fictional place perhaps that case doesn’t do much to balance the scales.