The first time I found a Team Fortress 2 thread on 4chan’s yaoi board , I was amazed.
Spy and Sniper on a couch, by Kob.
As an online multiplayer-only, plotless, mindlessly-violent first-person-shooter, I had assumed that the female fanbase would be limited; confined mostly to diehard first-person-shooter (FPS) enthusiasts, a subset of games with few female devotees. Fewer in number than female WoW players, or Harry Potter readers, certainly. Despite this, the vast majority of the TF2 fanart posters—and artists—seemed to be women. Weirder still, not all of them even played the game, and very few of them played the game regularly. This was stunning. It was as if I had happened upon a cargo cult—imagine someone showing up to a Star Trek convention with exquisite Spock fan art and a beautiful costume, having never watched the show.
Weirder still, the characters were drawn “realistically”, with scars, imperfections, body hair and normally sized penises—that last being incredibly rare, in what I’ve seen of the highly-idealized, manga-influenced genre of yaoi as a whole.  The TF2 images were shocking, just in how far they strayed from accepted standards of fanart—many of the pictures were humorous, charming, and deeply characterized. The goal of the images was the same as any other—to arouse, entertain, and/or titillate—but the methods seemed far removed from the oceans of anime-eyed Draco Malfoys stiffly fucking idealized Ron Weasleys, and the Dragonball gigantism foot fetish pinups.
I wasn’t on 4chan long before someone dropped a link to TF2chan.net—the imageboard forum where TF2 fans had set themselves up purely to produce and consume content about the game. And most of them, I quickly learned, were heterosexual women in their twenties. Which is true for the vast majority of fandom participants, across all franchises, as Henry Jenkins has pointed out. 
I was amazed. This was something different. The TF2chan posters appeared to be pickier, less forgiving. The images themselves—porn though they may be—were being held to a higher standard, giving a sense of production values to material that was usually treated with far less consideration.
What was it about Team Fortress 2 that inspired its fans, particularly young women, to produce material of this quality? Why was TF2, a first person shooter (which are typically not the domain of women), attracting such a large female fanbase?
Spy and Sniper, by Anadapta
Over my year of research (which is ongoing), I found that a large number of these fan artists are professionals, or art students going into the field. They are obsessed with the originality, grace, and individuality of each class in the game, and, on top of everything else, the humor. The outrageous framing, the perfect timing, the clever writing and professional voice acting in the official gameplay and promotional material is extrapolated, folded, stretched and draped, by fan artists, into a collection of pictures, comics, and stories that show enormous wit, creativity, and skill.
And the artists themselves were brutally critical. Anything drawn with the visual shorthand of manga iconography was dismissed out of hand as “anime trash”, or worse. Artists who posted their own work were subjected to corrections, critiques, and “redlines” (in which one artist draws over another’s lines in red, to demostrate mistakes in the original drawing). Artists were roasted, and either improved rapidly, or vanished off the board, back to DeviantArt mostly, where they wouldn’t be put under that kind of pressure. The focus on excellence in the art itself was far different than any other fandom I’d seen. 
Demo arm-wrestles with Scout, by vacant
I’d played the game, and enjoyed the accompanying official promotional material for years, so I already had a good handle on its exquisite characterization and art. What puzzled me now, was why this game specifically appealed to women, and further, appealed to women in a sexual way. Aside from the Halo franchise (which everyone played), Team Fortress 2 is likely the first person shooter with the largest female-to-male player ratio.  Why?
After months of looking at TF2chan, and then finding and following the fandom’s main contributors on Tumblr and other sites, I started to form a hypothesis.