I don't understand it either. Women have "busted" into the memberships of both Mens' clubs and Gyms via legal action.
Common sense would tell you it's not legal but....
http://www.eagletribune.com/news/stories/20010819/FP_003.htm
"Is it discrimination?
The subject of women-only facilities has not been without debate about whether the clubs violate the law and discriminate against men. In 1998, the debate took center stage in Massachusetts in court and at the Statehouse when lawyer James J. Foster fought to gain membership to a women-only Healthworks Fitness Center across the street from his Boston home.
The Massachusetts Commission Against Discrimination sided with the lawyer, but was overruled when the Legislature and then-Gov. A. Paul Cellucci approved a bill making single-sex fitness facilities legal. Three years later, Foster still considers the clubs discriminatory, but has no qualms with the state's ruling."