Does the "Dumbledore is gay" back story revelation discredit Rowling's Christian credentials? Not per se. It does, however, reinforce my judgment that this series is definitely adult fare. It should not have been written for children, nor should it have been marketed to children.
I had the same feeling about the Lemony Snicket books. The difference is that Daniel Handler definitely doesn't have a Christian world view, whereas Rowling appears to have something approaching it.
I think she sometimes isn't very learned or precise theologically or philosophically. And, yes, I think when you are dealing with the issues she deals with you need to be.