Some folks don't know when to STFU, JKR being one of them.
I do not doubt her that, in her mind, Dumbledore was queer as a $3 bill, or became so as his backstory firmed up over time.
Thing is, it is not appropriate to confirm such, given her target audience.
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Also, it is such a modern cliche.
A person (fictional or otherwise) can not just have little interest in sex, nowadays. There has to be some seedy backstory or reason why they aren't bumping uglies in public for all to see. A good example is J E Hoover, former FBI man. The lack of evidence of sexual activity on his part led to fools manufacturing a whole fictional cross-dressing/gay fantasy to satisfy THEIR needs. They just couldn't handle the thought of some folk not being slaves to their impulses.