Why the "Dumbledor was gay!" revelation is frustrating

I linked to that story a couple days ago, but I couldn't articulate exactly why I thought it was an annoying move on Rowling's part. Red Cardigan has some good points:

Truly great literature doesn't tie itself in such a slapdash way to an issue of the day...[She is not] playing fair with her readers to insert Dumbledore's gay-contemporary- political-identity into the works after the fact! This is the worst kind of cheating, when the author decides to pander to some political identity group when the final book has been published, and the readers think they know everything important about the characters that there is to know.