I definitely feel sympathy for this prompt's POV. As a reader of gen, het, and slash, I really don't understand why there should be conflict there. Perhaps because of my reading preferences, many of the writers I read repeatedly also write in more than one of these veins, so I have to wonder who it is who is having such problems with it?
My central thoughts about this topic were:
1) Can slash and het actually be called genres?
2) What tends to feed or block one or the other?
Regarding the first issue, I've seen the word "genre" used so often that I wanted to check what the actual definition is, and I was rather fascinated to see that genre and gender are historically related terms, as both refer to classes or groups of things. Amusing, of course, because gender seems to be one of the primary issues in these het/slash squabbles. Reading both, I don't find a great deal of difference in them for any other reason. I think there are more domestic concerns and babies in het fic, but it's called mpreg for a reason, and really, the author's approach and treatment of the story topics make more of a difference than who is having sex with whom.
Returning to what genre is, stories "having a particular form, content, technique" could be considered of the same genre. I don't know that there is any particular form or style that applies, as both het and slash encompass what I'd consider many other genres within them. Certainly there is a lot of overlap with het and romances, but I can say that an awful lot of J2 fic fits that category, as well as some slash stories, often AU, that I've read in the Buffyverse. Really the form and style varies not only by author, but often by story.
So if we're going to consider them genres it would have to come down, not even to content, but to central characters, and I'm not sure that the characters that appear in a story qualify it as a genre. For example, if we look at gay literature a lot of it deals with issues that affect gays as individuals and/or depicts stories with cultural aspects of gay life. Some slash stories do this too, such as stories that focus on a character struggling with a decision to be out or not. But I certainly wouldn't say that's content common to all slash stories, or which defines it in any way. In any case, that definition would make the stories gay lit and not slash. So again, what we have are two categories that encompass various genres.
Het and slash seem to me simply to be categories of fanfic. And I do think that fanfic is a particular genre. There are a collection of styles that tend to recur in it. The content always utilizes characters found in other stories with an emphasis on gap fic, and an exploration of characters' unexpressed, unexplored or underexplored emotions. This is true whether it is gen or romantic fic. It is also written for a particular audience, one which brings a considerable store of information to the stories, which are otherwise partial in nature. What I would say is that the most interesting thing about fanfic, as well as debates of het and slash, is that it is a genre characterized by people and not content -– whether we're talking about the characters that appear in the stories or the authors and audience who jointly create it.
I haven't been sufficiently involved with population-specific literature to be sure of this, but my assumption is that what makes it population specific is not just the inclusion of certain characters or writers, but the inclusion of particular types of conflicts, ideas, perspectives, and traits that add up to the depiction of a certain identity and cultural experience. For example, I would not consider a book featuring a female protagonist to be "chick lit" by default simply because its central character is female. While I suspect some men see any book featuring a woman to be inherently geared towards women, I don't know that women necessarily perceive it that way, and I quite doubt that they believe the reverse -- that any book featuring a man is inherently geared towards men.
However, from my own experience in reading fanfic, I don't know that I can say there is any kind of central identity or cultural experience being promoted within what are very varied texts. There is certainly an emphasis on central characters being outsiders, but this may spring from the fact that in many canon texts the central characters are outsiders. I haven't seen anyone do a study on it, but my guess would be that texts that show outsiders to be heroic are the most likely texts to have fanfic written about them. Of course, we can then quibble over what makes a character an outsider. For example, in Mad Men is Don Draper an "outsider" due to his background, which he certainly feels sets him apart from others? Because on the surface he seems to be the consummate insider (as Peggy noted just last week). More importantly, is he a heroic character? What he certainly is though, is secretive. That may be yet a third trait of characters seen often in fanfic, and perhaps one that many fans feel a connection to.
Moving on to my second point regarding where the conflict comes from, I'm going to guess some of it has to do with issues of canon. I suspect the sharpest conflict occurs around the (virtually always het) canon couples versus the (almost always non-canon) slash couples, and that in some ways the whole het vs. slash debate is just another version of "more canonical than thou" that always seems to find its way into every fandom. Until the last ten years almost all slash couples were by definition non-canonical, whereas all but the most unlikely het pairings always stood a chance of appearing on screen. For example, in Buffy fandom there was always a possibility that Anya and Giles might get together, even though it seemed unlikely in the setup of the show. Yet, sure enough, they are romantically paired in one episode. And while we do get several gay characters in the show, including one of the leads, the more obvious slash pairing of Angel and Spike gets a mere reference together with a Dumbledorian confirmation after the fact, making it easy to overlook.
So in some ways, I see the issue of slash versus het as a form of arguing for many more possibilities than we actually see in canon, something I think het stories rarely extend in the same way. To me, the very confusion that exists over whether a story is gen or het, particularly in a canon that is centered on romantic storylines, speaks to the ever present possibility of a het work being Kripked. So I think that one aspect of the het/slash divide has to do with producing more of the same from canon versus producing something largely different, even if, in terms of innovation, they're not that different at all.
For example, when I was watching Queer as Folk I thought it was interesting how storylines that could be considered clichéd in other shows had a fresh approach on QaF for the simple fact that gays and lesbians were the protagonists. A lot of the show's storylines dealt with issues that specifically affected queer folk, but others had something extra added to them simply due to the sex of the participants. For example in the first season, the fact that Justin's parents are not thrilled about him going after a guy pushing 30 is in some ways what we might expect from the parents of a 17-year old girl with a bright future ahead of her (which will probably be at a school somewhere else). But the situation is made all the uglier by Justin's father's homophobia. The stress upon his mother and sister lead to a more life-changing fallout than if the issue had simply been that of an unsuitable boyfriend. In the QaF case what the events brought out was that Justin himself was not what his parents had thought, and thus the whole family had to confront that as well, not simply his romantic choices.
What's interesting about SPN fic to me is that I feel rather the other way around about it. In that fandom it is the het which is subversive. It's not that we have an actual gay couple on screen, but I think it's telling that when Dean has slash explained to him in S4 he responds by asking Sam "They do realize we're brothers?" not "They do realize we're not gay?" While an actual sexual relationship between Sam and Dean isn't going to show up on screen, the entire show revolves around the constant push and pull of their unusually claustrophobic circumstances and the sizable emotional baggage each is hauling around. The only way to make it more fraught would be to add sex with one another to it, but in many ways the gen and the slash in that fandom are not that far apart.
However for SPN, het fic has to find gaps for itself because what few relationships the boys have been seen to have were short and tended to end poorly, and their relationships with women in general have not been particularly harmonious. To posit a het relationship for them, especially one that in any way resembles a more normal sort of life experience, is to turn canon on its head. A writer must constitute both a Sam and Dean who are significantly different from who we currently have, as well as a different sort of life for one or both. (Perhaps for this reason a lot of het is preseries or futurefic).
So in the SPN scenario I think we could see how a het versus slash struggle could turn on the issue of how canon is being skewed in order to make the stories work. I also think, considering that slashers and only slashers have represented fans in SPN canon (twice now), that there may be something to the idea that whichever fanfic form is prevalent in a fandom tends to characterize that fandom to outsiders. However, I think that same fan response speaks strongly to the canon content in the first place. Before I ever started watching SPN I had already gotten the impression the fandom was a slasher's paradise, something not unusual for buddy shows/movies/books. Without watching Torchwood at all I could have guessed the same for that show. In the Buffyverse, while there is certainly plenty of slash written, there have been voluminous amounts of het as well as a regular output of femslash. In the end, I think if fans want to place blame for the prevalence of het or slash in a fandom, they need look no farther than the original text itself.