Fitzhugh wrote two sequels, The Long Secret (1965) and Sport (1979), which maintain the gay symbolism. But in the 2000s, sequels by two other authors heterosexualize Sport. Harriet Spies Again (2002) by Helen Ericson, gives Sport a crush on a girl. And Harriet the Spy, Double Agent (2005), by Maya Gold, involves Sport and Harriet competing over the same girl.
One wonders why they heterosexualized Sport but not Harriet. Maybe there is more cultural anxiety about gay boys than gay girls.
Director Bronwen Hughes has also directed episodes of The L-Word and produced the movie Woman on Top (2009), which features a gay "best friend," so she is not unaware of gay/lesbian characters. Plus, notable lesbian actress Rosie O'Donnell plays Harriet's nanny, Golly.