There is no evidence for this in the books. Nor is there evidence for Albus and Minerva and Professor Grubbly-Plank having a menage-a-trois! In fact, ...
Well, JK Rowling said she thought of him as gay and deleted a line in a proposed movie script on that basis. What is remarkable is that there is nothing in canon to support such an attribution unless you consider that a celibate lifestyle in the face of a same-sex attraction (never mentioned or described or hinted) is JKR's idea of normative homosexual behaviours. And it is a viable lifestyle choice just as for heterosexuals.
Frankly, I'm glad we won't have any backstory torrid romances of either orientation!
Imagine:
Dapper Dumbly and Miss McGonagall Do Dallas!
Albus and the Boys from Brazil on Break!
Dumbledore and Grindewald Gone Wild!
Hogwarts Blanket Bingo!
Personally, it may well be that JKR came to that understanding as the storyline progressed over the seventeen years it's been aborning. Authors often speak of their characters having a 'free will' that precludes specific desired actions by the authors and which require the adaptation of the author to the character. Particularly, Dorothy L. Sayers notes this of her characters in the LORD PETER WIMSEY series. And we know that JKR likes DLS.
But for JKR to state such an understanding of her character's "self" does not imply that she gives a seal of approval to all that goes by that designation in anyone else's definition of what constitutes that "self". If it had been pertinent to the story line - and it is the delimitation of the modern obsession with sexual antics that her story line is most notable for by example (snogging as publically ridiculous) and by omission (none of the main characters are gratingly graphically so engaged) - she would have to have included it. It seems an afterthought.
Lest anyone say I am being too kind because of my great love for the HP series, I must add that to have revealed Albus Dumbledore in all of his mistakes and errors and Machiavellianness in DEATHLY HALLOWS and then make this revelation is not, to my mind, a resounding approval. Yet the positives of heterosexuality are amply displayed in multiple familial settings (even, yech!, the Malfoys! ... just how did little Draco get to be? ... don't go there!).
And this certainly didn't fit the speculations that specific characters might be gay : Lupin, Lockhart, etc.
Probably will spin off a rather tawdry round of fanfic, though!
You will find this essay of interest. It is about the fact that as entertaining as authorial comments may be, the canon is what matters.
http://www.scriptoriumdaily.com/200...han-the-author/