In early October, the photographer and Dallas-area native Fabian Guerrero got a curious message from some friends. They had visited the Amon Carter Museum of American Art, in Fort Worth, to see the newly opened exhibit “Cowboy,” which included Guerrero’s photographs of queer men in Western wear. But at its entrance, they encountered a sign. “Darn!” it said. “This area is closed.” Guerrero figured the museum was just conducting a private viewing. But then a curator emailed him to let him know that “Cowboy” had shut down for a week. According to Guerrero, the email didn’t explain why, other than to say that the show had reopened with a “mature content” warning, which included a QR code that guests could scan to preview the exhibit on…