But Peter Tatchell said the sexuality of the master wizard should have been made clear in the hit series of children's books.
Rowling "outed" Hogwarts headmaster Albus Dumbledore while appearing before a full house at Carnegie Hall in New York.
The award-winning writer's mega-selling fantasy series ended last summer with the seventh instalment, Harry Potter And The Deathly Hallows.
After reading briefly from the final book, she took questions from the audience.
One young fan asked whether Dumbledore, one of the series' most popular characters, ever finds "true love".
"Dumbledore is gay," the author replied to gasps - and applause.
Gay human rights campaigner Peter Tatchell said: "It's good that children's literature includes the reality of gay people, since we exist in every society.
"But I am disappointed that she did not make Dumbledore's sexuality explicit in the Harry Potter book."
Rowling explained to the Carnegie Hall audience that Dumbledore had been smitten by rival character Gellert Grindelwald, whom he defeated long ago in a battle between good and bad wizards.
"Falling in love can blind us to an extent," Rowling said, adding that Dumbledore was "horribly, terribly let down".
She said the character's love for Grindelwald was his "great tragedy".
Potter fans had previously speculated on the internet about Dumbledore's sexuality, noting that he had no close relationship with women as well as a mysterious, troubled past.