Next month marks the 40th anniversary of two landmark events of American popular culture: the assassination of pioneering gay politician Harvey Milk, and the mass suicide of 900 members of the Peoples Temple cult. If you remember anything about the mass suicide, it’s probably that cult leader Jim Jones was a fundamentalist Christian demon whose Bible-thumping berated brainwashed followers into drinking cyanide-laced Kool-Aid. Harvey Milk, by contrast, is a folk hero. The San Francisco supervisor became a gay-rights saint because he was martyred by a right-wing fanatic. That’s the received history in both cases. But according to author Daniel Flynn,...