Why are There No Males?Edit
One theory suggests that a disease that only affected those with a Y chromosome befell on the Earth years ago, thus eradicating the male gender. Since then, all information on this catastrophic event has been suppressed. Another theory posits that there was a mass genocide of men at some point that was so terrible, that, like with the first theory, all information of the genocide was withheld.
A slightly less dark theory sees the cast of Yuru Yuri as being aliens on another planet. According to this theory, the race of aliens look feminine in appearance, and they also have a town that is referred to as Japan. Or, the cast could live in a parallel universe in which males do not exist.