I'll censor myself a little as not to sound accusing, especially with so few smilies at hand.
I have trouble understanding why anyone with writing talent and abilities and appreciation for Tolkien's words to boot would want to slash Tolkien's characters. I mean, why not create your own characters and slash them? If you agree that slash is not cannon then what you have is out-of-character writing: the characters may bear Tolkien's given names but they not their personality. In other words, why not create your own story, set in a fantasy world to explore the implications of slash on identity crisis, etc. Sorry of I appear to be picking on you particularly, Maril, but I am actually expressing a dissatisfaction with the slash fanfics I read. The badly written ones posed no problem to me as I left unfinished, the well-written ones were much worse because they seemed a paradox: it's plausible but it's impossible. Perhaps I should also add that cannon means a lot to me: Part of the charm of LOTR is that it's written by Tolkien: he was a talented, very learned man, and he has the ultimate say when it comes to his creation. At least that's how I see it. That's my opinion about all non cannon writing, be it slash or other unusual pairings.
About slash specifically, and why so many people are trying to reinterpret great works as containing same-sex relationships (especially when it's clear that the writer did not intend it) - I think it's part of this whole campaign of making homosexuality acceptable: let's associate it with a popular thing, even if it's only for laughs, people will come to tolerate gays more like this. Well, I don't know about others but I find myself much more tolerant toward gays when these tasteless jokes are not being made. If anything, it has the opposite effect...