I can tell you who I’m writing for, and I can tell you who I’m keeping in mind,” romance novelist Casey McQuiston says. We are one cocktail in at the Scarlet Lounge, an Upper West Side bar co-owned by the actor Michael Imperioli. “I am writing for trans people — capital-F For,” McQuiston says. “Trans people, queer people, those are a lot of the people who engage with my work in ways that make me feel like they got it.” But McQuiston, blockbuster queer-romance author of The Pairing (out in August)...