TEM, she did leave it to the imagination until the moviemakers tried to mess up her view of her creation and then someone specifically asked her about Dumbledore's love life in a public forum.
Totally fair, if you ask me, since the issue was pressed by other people both times.
Since she was talking about movie 6, you could also say that Dumbledore is only gay in the film universe, and assumed straight or unknown in the book universe.