|See also:||Bob (genre), Satire, Story Tropes, Gen, Het, Slash, Femslash|
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It can also refer to other sexual, relationship or character-based elements; topics such as polyamory, threesomes, trans characters, or kink (especially BDSM) are often considered to constitute their own genre.
Fanfiction also includes many genres rare outside of fandom, such as anthropomorfic or Mpreg, and many fandom-specific genres such as Muldertorture. Fandom-specific genres may spring up in reaction to canon events, such as Harry Potter Epilogue? What Epilogue? fic or Blake's 7 Post Gauda Prime fic (both subgenres of denialfic). Other fanfic genres are related to the structure of canon; for instance, post-eps are common in episodic fandoms which make heavy use of the reset button, and the casefile is usually found in a police procedural fandom.
Differing Opinions/Changing Landscape
Fannish opinion regarding gen, het, and slash is a moving target and one with a long history.
A fan explains some history behind the term "gen in a 2014 discussion at Fail-Fandomanon:
[Gen] originally stood for "general". You had slash zines and general zines. General, at that point, also included het, though the het porn was usually categorized as "Adult" - but it was still filed with the General stuff.
Sometime in the early/mid 90s, General and Relationshipper split off from each other. At that point 'shipper exclusively meant het. Slashers were not 'shippers. So you had General, Shipper, and Slash. (I think the split came in X-Files fandom, which invented the entire term 'relationshipper', which is where we get ship from, and had the Mulder/Scully vs No Romance wars, but don't quote me on that.)
Then, in the early 00s, 'shipper lost it's apostrophe, lost it's -per, and just became "ship". It stopped meaning exclusively m/f, and started meaning all romantic relationships in fic, which is when we start calling things "het".General, which had long since lost it's -eral, therefore became exclusively about non-romance sometime in the mid 90s, because the romance camps had so firmly split themselves into separate categories by that point.
Some anecdotal opinions:
I hate the term 'het' anyway. For me, they're either sleeping together or they're not. If they are, it's slash. If they're not, it's gen. At least, that's how we did it in the old days....
I can understand the het label if the story is primarily about sex. But I consider relationships with people and yes, even sex, part of everyday life. Even in a gen story, there might be a romantic or sexual relationship although it wouldn't be the point of the story.
Alternatively, I've read slash stories in which there was no sex.
So, I'd just rather see a pairing indicated (or none) and a rating. If it's R or higher, slash or gen, there's sex or strong violence. I'm not a fan of complicated labels.I know there are folks who don't agree but I really can't fit my stories into narrow boxes, especially the longer ones. 
...the definition of gen - [is] not that sex doesn't happen, but that a pairing is not the focus of the story. I have a brain like a sieve, so I can't remmeber where I originally saw the "gen isn't shippy" definition, but that's it in a nutshell - it's not the gender of those who are romantically/sexually involved that counts, but the extent to which romantic/sexual involevment is the point of the story. So for readers like me, the most useful distinction an author can make is not slash vs everything else or het vs everything but gen vs everything else. I realise, though, that other readers have very different priorities, and I think the three-way gen/slash/het distinction is the most workable solution (although these days, when most fic has a header with pairing details, specifiying the genre is often redundant). 
- You're Messing with My OCD, Fandom. You Wanna Fight?, Archived version by viciouswishes (January 11, 2007)
- recs, and meta, Archived version, post by cofax, March 21, 2007
- on fanfic, genre and categorization, Archived version by sholio (March 22, 2007)
- Gen, Slash, and Het: Warnings or Genres?, Archived version, alixtii (March 27, 2007)
- I got momentarily sucked into one of the "Ew, gross, there's ship in this gen!" conversations via metafandom the other day, ..., Archived version, Barb (March 29, 2007)