Last week, second-graders from San Francisco's private all-boys Town School strolled through the Castro, the city's world-renowned gay district where a huge rainbow flag flies gloriously above Harvey Milk Plaza. The group of about four dozen boys, ages 7 and 8, was led by a reputable local historian who regularly leads walking tours of the Castro (and who asked to remain anonymous in this story). The guide walked the boys by Pink Triangle Memorial Park, where 15 granite pylons rise above the ground in remembrance of the estimated 15,000 gays, lesbians, bisexuals and transgenders who were persecuted, imprisoned and killed...