When we look for Serenity in the DVD store we look under "Science Fiction" (if the store has an S.F. section), because the movie is more science fiction than it is a western. You won't find a sticker on it saying "Warning: This film contains Western elements" and even if you read the entire back cover you won't find any indication that some people consider the film to be a western in addition to being science-fiction (in part because the cover sucks, but what can one do). (I'd use Firefly rather than Serenity to make my example, as the former has more western elements than the latter, but it'd probably more than likely be found in a "Television" section--which shows that fannish categories don't make any less sense than any others.)
If I go to the bookstore and browse through the science fiction section--which I do less than I used to, since I have an insane number of books already purchased and unread--they are in that section because they belong to the genre. They do not, however, have warnings that say: "Warning: This book may contain mystery elements" or "This book contains a boy and a dog" or "This book has a romance in it." No, it gets filed under the dominant genre, the best fit. The summary, if the book has one (not all do), might give a sense of the secondary or tertiary genres if there are any. (And even then the summaries aren't written by the author, so sometimes the summaries suck and give away too much, and sometimes they seem to be for a completely different story altogether, like the summary on the back of my copy of Time Enough for Love.) Then again, it might not, and you might find out half-way through the science fiction mystery you've been enjoying so much so far that it also includes a romance and a boy with a dog. If you hate these elements so much you can't go on, then tough, you're out $6.50 you could have spent on a different book.
In the fanfiction world, things are much better for the ridiculously fragile: all fanfics come with a money-back satisfaction guarantee.
"Het" is a genre which includes texts that focus on m/f romantic or sexual relationships. It is not a warning; I refuse to label my stories for someone so ridiculously fragile they will be crushed if they come across a reference to a heterosexual couple, even (what they, using their hermeneutic, consider to be) a non-canon couple. "Femslash" and "m/m slash" are genres as well, not warnings. I absolutely will not label my stories for someone so ridiculously fragile (or homophobic, although in these cases I don't think my interlocutors are) they will be crushed if they come across a reference to a homosexual couple, even (what they, using their [heteronormative] hermeneutic, consider to be) a non-canon couple.
Indeed, the best way one can tell that "gen" isn't a genre the way the gen fans (or at least the vocal gen fans with whom I've been disagreeing) have been using it is that it can be defined far too precisely. Genres don't work that way; their edges are always-already fuzzy. Warnings--or, in this case, the lack thereof--do. A genre specifies what a work is about, whether it's about falling in love or solving a mystery or fighting demons, which is subjective. A warning specifies if a given element is present--think of those warnings for peanuts on products that don't even include peanuts, because people can be just that sensitive to the oils--which is not subjective.
"Milk Chocolate" M&M's may contain peanuts; it says so on the wrappers. That doesn't make them Peanut M&M's, and anyone looking for Peanut M&M's and finding Milk Chocolate ones isn't going to be satisfied.
Anymore, the only time I warn, ever, is for non-con. I wouldn't (I don't think--I'm not making any promises) withhold pairing information if the relevant pairing was incestuous, even if it was very brief, and I think I might even add a note in the case of a pairing like "Cindy Mackenzie/Lauren Sinclair" making clear the relationship, so you could say I'd warn for incest. But I don't warn for death, and I don't warn for pregnancy, and if I've already said it, I don't care, I'll say it again: I will not warn for het or femslash or m/m slash. Absolutely out of the question.
I provide a Genre Index so people can find the sorts of fic they'll probably like, like putting all the science fiction books together in the bookstore--not so they can be protected from stuff they don't like. If you like homoerotic stories about women which are romantic and/or sexual, you'll probably like the stories I classify under "femslash." But that doesn't mean you won't find elements you don't like there. Tough. As I said, money-back satisfaction guarantee. (Stealing a languagelog chestnut even more, I'll even throw in a free year's subscription to this journal.) Stories are frequently listed under more than one genre, and yes, a story about fighting demons in which Dawn and Giles just happen to be married will be listed under both gen and het. (And I suspect the reader going in expecting pure het is going to be more disappointed than the reader expecting pure gen.)
Admittedly, it's not just gen fans who want warnings when it comes to pairings. 'Shippers can be just as bad, with a Buffy/Angel fan not wanting to hear a mention of Buffy/Spike or Spike/Angel even if the fic is post-"NFA," or some such. A plague on both their houses, I'll say--if a couple paragraphs referencing some (in your mind noncanon) pairing in a long plotty multichapter epic can ruin your entire reading experience, you really have to get over yourself.
But this is the first time I've heard anyone suggest that we subvert the entire genre classificatory system to turn it into a warning system. Because just, erm, no.