A tendency in some Slash fics to make characters from the original work come out in an improbable quantity. It is especially prevalent in fandoms of things with a Cast Full of Pretty Boys. Bishoujo Series and series with Improbably Female Casts are also fairly susceptible to this with the other gender. Often a form of Canon Defilement and Character Derailment.
In stories, movies, and games that have only men in the cast there's virtually no choice for shippers to avoid this route, other than Gender Bending or adding in their own original character.
Often occurs in Yaoi Fangirl circles which think that "Het Is Ew", so that almost all male characters find themselves in same-sex pairings. Unfortunately, within these circles, female characters can fare poorly as those who don't end up becoming Fag Hags or coming out of the closet themselves end up suddenly evilor dead.
This occasionally happens in sanctioned adaptations and in that case, it's Cast Full of Gay.
Aforementioned major female character, Hungary, is a complete yaoi fangirl who had to debate whether or not to rescue her then-husband Austria when he was being molested by France.
But as the female nations appear more and more frequently, it's becoming less and less justified...
Azumanga Daioh: Somewhat justified because not only there are quite a lot of Les Yay going on, there are only two male recurring characters, one being a Dirty Old Man (who despite all reason is married) and the other a floating mythical cat thing who may or may not be the father of one of the girls. Neither one makes a good Ship outside of Crack Pairing.
Fans of Ranma ½ pairings are very quick to ignore Ranma and Ryoga/Mousse's sexualities and pair them together... that is, when they're not pairing Ranma up with any of his other various suitors or Nabiki, and Ryouga with Ukyo or Akari (for manga readers). These pairings also seem to disregard Ryoga being madly in love with Akane (and then Akari) and Mousse likewise being fixated on Shampoo.
Code Geass. Despite that over half the male cast is motivated by an obsessive love for a member of the opposite sex, fans continue to ignore the canon and ship Lelouch with Suzaku, Lelouch with Rolo etc. Then again, everything CLAMP touches (they did the character designs) tends to turn to Ho Yay, for better or for worse, with one exception
To be honest, Code Geass is not the worst example, Suzaku/Euphemia is quite a popular ship, and so are Lelouch/C.C. and Lelouch/Kallen. On the other hand, canon gave us...Kanon. Whose very first line implied he was Schneizel's boytoy.
Get Backers. There are a few women, but they are often shrugged off as "Ban's sister", "Ban's surrogate mom", "too old & too tall to be with" & "hologram tomboy". The rest of the cast is guys, some with very die-hard clingy relationships, & Akabane who only hits on a couple of guys while showing no interest in Hevn.
Gundam Wing, in complete defiance of the show's 1-to-1 gender parity (and the fact that about half those man/woman pairs are Official Couples or extremely close to it). Rarely this also applies to the women, usually by authors too kind-hearted to give them the usually treatment. This isn't helped by the director supposedly saying "I can't write boy-girl romances" (he actually said he couldn't write romance period, not distinguishing boy-girl from boy-boy, boy-robot, and robot-humongous centipede) in an interview (though he also said that didn't mean it was impossible).
All Gundam gets this, pretty much. Even the ones where the characters are paired with (in Kira Yamato's case, actually have sex with) women...
In certain older circles, Amuro and Char from the original are considered to be the first couple of yaoi. And that's not even mentioning Char's Ho Yay with Garma Zabi...
Hunter x Hunter. Many of the boys are pretty, friendships range from overprotective obsessed friend to acts-like-an-old-married-couple, there's a pedophile, very few women, & lots of fanservice pandering in the live musicals.
Especially since Tsumugi is heavily implied to be a lesbian...and the "relationship" between Mio and Ritsu is kinda similar to the popular Lucky Star pairing "Konami" (Kagami/Konata).
Monster. Never mind that Tenma was engaged to a female. Nevermind that Grimmer had a wife and a son. Nevermind that Runge has a daughter. Nevermind that none of them, in any possible light, look remotely bishonen (except for Johan, but he's, well, you know...)
Rather helped by the large numbers of male characters in the cast. Take Prince of Tennis for example. Female characters in the manga with more than a page or two of screentime? Four in the main age group; you can push half a dozen if you include the adults. Male characters? Literally in the triple digits.
The Pokémon anime. Forget that the main character is 10 years old, let's write some "lemon" involving him and his arch rival getting freaky! And hell, we'll even throw in Pikachu, a Master Ball, and a bottle of ketchup to spice things up!
Saint Seiya. There is no limit to the gay pairings in that one, since, well, about 95% of the characters are guys, and about 90% of them look effeminate in the manga. *
e.g Camus' has prominent eyelashes and what looks like nail-polish & lipstick
Never mind all the Bronzies are half-brothers. Everything's possible, even twincest with characters like Kanon and Saga who are hugely popular.
The anime also added its fair share of fuel to the fire by boosting the androgyny of characters like Lizard Misty and Aphrodite Pisces several levels, and by adding gratuitous instances of male fanservice.*
Weiss Kreuz, or at least, that's what 90% of the Yaoi Fangirls like to believe. The former bastion of all shonen-ai fandom, with an extensive male cast (at least 8) to form manycrack pairings out of. Then The Prince of Tennis came along...
Yami No Matsuei. You can't throw a rock in that fandom without hitting a slash fic.
Given that there are only three or four female characters that play a somewhat bigger role and/or appear more than once, shippers don't have much of a choice.
Yu-Gi-Oh!: The four main pairings are Yugi/Yami, Yami Bakura/Bakura, Kaiba/Yami and Kaiba/Jounouchi. Thanks to Yu Gi Oh The Abridged Series, Yami Bakura/Marik is rising in popularity. Quite often, all those pairings (particularly the so-called "Yami/Hikari" pairings) appear in one single story.
Also happens a lot in the Black Butler fandom, because of all the Ho Yay. But seriously people, statistically at least one of the male characters has to be straight.
The iDOLM@STER to the point where some fans were enraged that a male was put into the show as the producer. Being the only male in an otherwise Improbably Female Cast however means he can still be largely ignored.
Slam Dunk: Never mind that the protagonist starts playing basketball to impress a girl, 99% of fanfics and fanart for the series is slash.
Pani Poni Dash! characters Kurumi Momose and Sayaka "#6" Suzuki sure show their love for each other in both respective Twitter accounts that they would get married. While there's no romantic fanart yet, the PPD fandom is slowly starting to notice it.
Corollary: Any manga/animé with at least one Bishounen, Cute, [[Moe Moe]], etc character. Which is to say, Any Animé and Manga. (Ok, there is maybe a 0,2% that doesn't)
The Lord of the Rings. That's because the main characters are four hobbits, an elf, two humans and a dwarf. And they are all male.
Peter Jackson's film adaptation of the LOTR trilogy seems to magnify the Ho Yay inherent in the situation.
That also goes for The Silmarillion. Again, there are many more male characters...oh, many are supposed to be married, but often enough JRRT doesn't even give us the name of their spouse.
About half the fanfics of the Jeeves and Wooster stories only feature Jeeves and Bertie anyway...
Warrior Cats. In the books, mates are strictly straight, and Word Of God has said that she will never touch the issue of homosexuality. But look on the fanforum? Ohh man.
Wouldn't this be a case of Everyone is Bi, since most major characters have an almost equal number of slash and het pairings attached to them?
Blake's 7. One of the most-slashed shows ever, mostly because the show deliberately avoids making the characters' motivations 100% clear, so you never know what they're thinking and who they are really attracted to.
The Daily Show. It doesn't hurt that Jon has described it as "the gayest show on television".
It may or may not help that Claire is now almost officially in a relationship with her bisexual roommate.
In Star Trek: The Original Series fandom, the ENTIRE main cast has been slashed together, no matter how weird the pairings have to be in order to Tie Up Romantic Loose Ends. The two main female characters, Uhura and Chapel, are shipped together just to even things out.
Bear in mind this is the fandom that created slash...
Happens more and more often in the Power Rangers fandom these days, but Het Is Ew only comprises about half the fandom's opinion, so there's still a wide range of fic. For now.
Merlin could be considered justified due to the insane amounts of subtext in the show between almost every character, resulting in this being one of the most peaceful conflicting-ship fandoms out there.
iCarly has fics with a pairing of Carly/Sam, where everyone comes out as either outright gay, or bisexual. Apart from the obvious Sam and Carly, Spencer often tells the girls that he is bisexual, usually with his best friend Socko, Wendy is often made gay so one of the girls can go out with her as part of the story's drama. Freddie comes out and admits he has a crush on Spencer as well as admitting his crush on Carly is overcompensating to look normal.
Victorious can have this done, so that Jade can be hooked up with either Cat or Tori, whilst Beck and Robbie give in to their obvious UST and Trina just goes both ways because she's crazy like that and it lets Cat hook up with her if Tori ends up with Jade. Weirdly, Andre often gets left as the sole straight character.
Happens so often in Glee that Kurt and Rachel have been paired with pretty much every male or female, respectively, that have existed in the show. Seems to be THE RULE for most fanfics. Granted, Santana, Brittany, Kurt, Blaine, and now Karofsky are all gay/bisexual/bicurious.
Supernatural. Dean/Sam. Dean/Castiel. Sam/Castiel. Dean/Sam/Castiel. Dean/Azazel while possessing his father. This show, with Shipping Goggles on, is one big gay playground. And don't even get me started on the Actor Shipping; a lot of fans truly believe that Jared, Jensen, and Misha's wives are all beards.
It's actually ridiculously easy to do. What with all these big oiled-up hard bodies in very few clothes rolling around together in compromising situations and probably something like a 10:1 ratio of men to women...
Mega Man and its other series have a cast that is mostly 99% male, making this pretty much inevitable. Can be averted if you think that robots wouldn't care about or even have sexual orientations anyway.
This is most certainly not assisted by the works of Hyadain for 2, in which virtually every Robot Master is gay for Rock. Crash! Let's do it, indeed.
Much like the anime, the Pokémon game fics often pull this. The lead male and the (male) rival are very commonly paired off, with the lead female- when she does appear- being paired off with another female (for instance, Hilda and Bianca).
Fans of comics in The Walkyverse have a saying: "A character is assumed to be gay unless proven otherwise. And even then, it's suspect". It's mostly a joke... mostly.
Basically every character in Concession is gay, bisexual, transgendered, a pedophile, or any combination of the above. The author himself is bisexual, and seems to have a warped view of sexuality in general. To make matters worse, this was played for laughs (even the pedophilia) for the first 70 issues, then suddenly made very morbid with the onset of cerebus syndrome.
For Eternity, a web remake of Rainbow Brite, has all the Color Kids (as well as its versions of Rainbow Brite, Stormy, etc) as male. Considering that the only females in that universe are childlike temple guardians or the four mother Goddesses, this is justified.
The most recent video on Jandrew Edits has taken their TNG video edits directly into this, by bringing LaForge into the existing Triang Relations between Picard and two Rikers.
Red vs. Blue slash fic often goes along with this. Partly because there are very few female characters in the series, even if you include the tank, and all of them are either dead or missing.
Homestar Runner fandom tends to do this quite a bit, although considering that the only girl resembles a broomstick, doesn't get along with her canon boyfriend very well, and is the textbook definition of a Granola Girl, this is perhaps understandable.
The Penguins of Madagascar uses this in canon and fanfiction. All the penguins have ambiguously gay moments, and we're still not sure if one or more of them is female. Meanwhile King Julien is almost certainly Bi the Way. Short of transferring OC female penguins, or going with Anybody/Marlene, gay is your only choice.
Metalocalypse, with its nearly all-male recurring cast, is ripe for this trope. While all of Dethklok does show interest in women (specifically, ladies), and Skwisgaar and Nathan actually shown in bed with a few, all of the notable women they date look like the other band members. Combine that with the mountains and mountains of gay innuendo, the band's canon relationship as True Companions, and the rampant substance abuse in the show, it doesn't even necessarily have to be a matter of Everyone is Gay as much as No One Is Straight.
South Park probably because there are so few important girl characters other than Wendy and Bebe.
In an odd twist, every one of the boys (including Butters and Cartman), have shown attraction to woman and many (including Butters) have stated that they're straight. Everyone is Bi, maybe?
If Transformer reactions to Blackarachnia are typical, there's definitely some kind of sexuality in there. So stage one is also reasonable.
In the Beast Era, anyway. In the Machine era (read: most of canon), not so much.
There are also, across the various series, canonical references to descendants and ancestors, siblings, parents, etc, making the last reasonable, as well.
Unless you're talking Animated, where it's specifically denied as they reproduce via the Allspark, G1, where they reproduce via Vector Sigma, or the live action movie, where they reproduce via the Allspark. So, basically, it's only feasible to have in RID and the Unicron trilogy.
Could also happen in the movie verse as well - since the Allspark was destroyed, where else could all of those protoform eggs have come from?
Kim PossibleFan Fic is crazy for the Les Yay, especially with Kim, and particularly with Shego. But then, there aren't many decent male characters that aren't family or trying to kill her.
And when that happens, out comes the male slash counterparts. Drakken/Ron/Felix, Monkey Fist/Ron, Killigan/Monkey Fist, Electronique/DNAMY, Ron/Brick, Motor Ed/Drakken.
It may be more than just a shortage of males; the comments these fanfics and fanarts get imply that a fair-sized chunk of the Periphery Demographic thinks Girl on Girl Is Hot.
This trope also leaks onto the boys, especially Big Mac and Caramel.
Due to the team mostly being very attractive guys, Young Justice fanfics are full of this. The most popular seem to be Robin/Kid Flash, Robin/Superboy, Robin/Red Arrow, Aqualad/Superboy, Aqualad/Red Arrow, and Kid Flash/Red Arrow.
Cabin Pressure. With a core cast consisting of three men and one older woman (who is moreover the mother of one of the men), what else are frustrated fan fic authors to do?
According to the comments sections of any given celebrity gossip blog, every attractive male celebrity is gay. No exceptions. If he has a girlfriend, she's The Beard. If he's seen in the company of another male more than once, this other man is his secret lover. If he marries a woman and even gets her pregnant several times, it's a calculated move to protect his career by appearing heteronormative (which gives us the bonus Unfortunate Implications that all famous closeted homosexual men are perfectly happy to callously use other human beings, even their own children, for material gain). If he has stated on the record that he is actually bisexual, he defaults to gay because male bisexuality doesn't exist. If you point out that there is nothing beyond unfounded gossip to indicate his sexual orientation one way or the other, you are automatically a fat, ugly fangirl who is deluding herself and the sole reason gays are terrified to come out of the closet. Not surprisingly, no one ever questions the sexuality of, say, Steve Buscemi, Ernest Borgnine or any other male performer who is not conventionally attractive.
Read the page for Ambiguously Gay. Half the examples on that page are confirmed to be canonically straight but listed because they have only one or two of the qualities required.
