Well, I'm of the "doesn't matter" school, and I'm not certain it's a knee-jerk reaction (or if it is, second thoughts don't contradict it much).
I tend to have the same reaction to people's sexuality, I'm not extremely interested in what happens in their trousers. "You know what, he is gay
" sounds a bit similar to "you know what, he is Jewish
" to my ears - it makes me wonder why it should be relevant, and to what, to the eyes of the person who triumphantly conveys the info.
I believe by default that Tintin is heterosexual(ly asexual), because fiction a domain where I gladly accept Occam/ Akham's razor to cut out what the author didn't specifically hint at. Hetero was default, I doubt Hergé had the intention to make Tintin particularly homo. That said, Tintin is hardly more than a smiley (as I claimed elsewhere, comparing him to his more defined, fleshed-out and ultimately likeable companions).
He's, like most comics heroes of that era, some Rorschach blot, allowing anyone to project his/her identity onto him - hence his huge success from leftists to Nazis, and the distrust that it sometimes leads to.
So, projecting one's homosexuality on Tintin's undefined sexuality is as legitimate as projecting one's own voice. Tintin is a blank piece of paper. I just believe that in our current culture (and in particular in that of Hergé), blank papers are heterosexual, white, Christian and male, before you start drawing on them. And the unpainted bits tend to remain so.
That said, I disagree a bit with :
There's no reason to think that Tintin would lack the hormones of someone his age.
as I think there's no specific reason to believe in hormones in the universe of old French/ Belgian comics. Puritanism excluded boy/girl matters from these books, generally producing worlds where girls were rare (blank page default=male), and characters de-sexualized.
Means, intercourses, hormones, procreation, women, didn't exist in that universe (unless some Gargamel builds up a Smurfette). All these questions about Tintin or Spirou's sexuality seem out of place, to my eyes.
Their world is not exactly ours, it's more similar to the world of childhood and all the things that just didn't exist yet. So these questions always seem to me to be bringing "outside questions" into their universes ("How does Spirou earns his living?"; "How many taxes does Tintin pay?"; "How long is Calculus's wang?"). Which is an adult pass-time I'm not too much a fan of.
To my eyes, wondering about Tintin's sexuality makes as much sense as wondering why Wile E. Coyote wants that particular Road Runner so much, and where that Road Runner is going.
Edit (some sort of double post):
I do think Tintin could well be gay, but I don't particularly want to get into a discussion about it because of the vitriol from those who strongly think that he's not.
Actually, I think that any discussion subject that makes homophobes hysteric is awesome by definition. So this would rather be an argument against what I've typed above.
Also, just found this in the Sadoul book. They're talking about the lack of women in Tintin books. Numa Sadoul asks if it could be due to some latent homosexuality in Hergé himself. Hergé answers :
"I don't know. One never knows, does one? But I don't think so. If I had homosexual tendencies, I don't see why I'd hide them."
Hardly the answer of someone who'd feel "insulted" by the question, right? And even less, I suppose, if asked about his characters. I think the answer can be applied to Tintin pretty directly (except that, in Tintin's context of publication, one could see why he'd hide it).