To combat inequality in society with gender, sex and sexual orientation, the government has imposed a law where everyone has to spend one year as the opposite gender. They have between the ages of 16 to 41 to do it, and if they don't by the time they're 41, they spend the year in prison. (On reason that if you can't pull your effort to better the society that way, you'll be removed from society and better'd that way?)

John did his at some point pre-show. Sherlock is fast approaching 40 and has yet to do his year.

Extra: There's no obligation to turn back to your birth assigned gender after that year.