His clothes – always the flashy dresser, Dumbles like to walk round in a purple cloak, the very height of magical fashion. All he needed were some high heels and a handbag, and he’d be sorted. In fact, there’s nothing in the books to suggest that this isn’t how he dressed in private.
He's bad with directions – Apparently he has a map of the London Underground on his knee; I can’t remember if the average gay man is meant to be good with directions or not, but I’ll go the safe option and say they're rubbish. So yeah, this is solid evidence, right here. And didn't he always get lost in the castle?
Collects jewellery – He gets his hands on a pretty ring which he knows is probably cursed, but it looks so pretty he can’t resist! He puts it on, gets cursed, brings about his own downfall. That says a lot about a man, if jewellery is more important than saving the World, doesn't it? Oh, one more thing - in his bank vault there is only one thing: a giant gemstone. Flashy!
He's a former head boy – All head boys turn out to be either gay or bi-curious.
He has a long beard – And he obviously gives it a lot of care; bathing it in whatever the magical equivalent of Pantene is and brushing it every night. How else could he keep it so shiny and soft? Anyone who grows a beard that long is gay – just look at ZZ Top.
When he looks in the Mirror of Erised he sees Christian Bale naked – Yeah, this one was never made completely clear, but it’s obvious now you think about it. Dumbles was obviously lying about the whole “socks” thing, and the only reason he’d do this is because he felt embarrassed that he fancied Bale pre-Batman.
He likes raspberry jam – The gayest of all the jams.
He didn’t think Hermione was hot – The worst crime of all. In that last Harry Potter film, the entire male cast is checking out Emma Watson for the entirety of the showing, even the teachers. But have a guess who was the only character who wasn’t seen mentally undressing her? That’s right – Dumbledore.