June is Pride Month, a ripe time to reflect on one of the most startling facts about our sexual culture today: Condom use is all but disappearing among large numbers of gay men. Many rightly attribute the condom’s decline to the rise of PrEP — an acronym for pre-exposure prophylaxis, a two-drug cocktail that inoculates a person from contracting H.I.V. But another crucial component is the fading memory of the AIDS crisis that once defined what it meant to be gay. After tracking the sexual practices of 17,000 gay and bisexual Australian men from 2014 to 2017, a team of...