To most ears, it probably sounds inoffensive. A little outdated and clinical, perhaps, but innocuous enough: homosexual. But that five-syllable word has never been more loaded, more deliberately used and, to the ears of many gays and lesbians, more pejorative. “ ‘Homosexual’ has the ring of ‘colored’ now, in the way your grandmother might have used that term, except that it hasn’t been recuperated in the same way,” said George Chauncey, a Yale professor of history and an author who studies gay and lesbian culture. Consider the following phrases: homosexual community, homosexual activist, homosexual marriage. Substitute the word “gay” in...