Because there is no situation in which the suit presented in your hypothetical would be successful. The military would either point out that the person(s) filing the complaint knew they were agreeing to share same sex accommodations when they signed up to serve, in which case the complainant would lose, or it would be a case of sexual harassment and the individuals responsible would be pursued for creating a hostile environment. There is simply no situation in which it is even remotely reasonable to believe that someone would be successful in forcing the military to provide private showers, barracks, and dressing rooms.