Revolutionary Girl Utena: The TV series contains suggestive scenes and dialogue between nearly every pair of major characters, including siblings and completely regardless of gender. The director kept pushing the envelope until not even the most diehard of Have I Mentioned I Am Heterosexual Today? fans could call it "just Ho Yay". Even the manga artist, initially disinterested in the plan for girl-girl intimacy, came around near the end of the series. The brakes came all the way off for the The Movie, where the main couple went from the heavy subtext of the series to unambiguous lesbian romance.
Seems to have been the general assumption in Kyo Kara Maoh, as it is 'not uncommon' for two men to marry, and the maids who have a betting pool on the main character's relationships don't even take gender into consideration despite his having explicitly stated his heterosexuality several times. Or perhaps they just know better.
It seems like absolutely everyone in Axis Powers Hetalia is at least bi, which is pretty much what happens when 90% of your cast is male but you still want romantic/sexual tension between the characters. At the very least, Word Of God has confirmed that France is bisexual and Sweden is gay for Finland.
In Mnemosyne, the entire issue of gender in sex is just discarded. For example, the main character Rin has shown to have sex with both males and females, with the one person she's loved for over a millennium being the guardian of the Tree, and the rest of the cast is implied to simply go with what feels good to them. That said, however, gender is an issue to the greater storyline... well, actually it's rather the mix of gender that's important. The point? The only character with real tangible power in the entire anime is an hermaphrodite, Apos, at least until Rin becomes the new Guardian.
Seems to come with badge if you are a male police officer in FAKE.
In From The New World, all of the main characters except Mamoru. When they're 14, their entire class (save him) appear to be in a relationship with someone of the same sex. It's also a case of No Heterosexual Sex Allowed at a young age.
Wendy and Richard Pini, creators of ElfQuest, have explicitly stated on several occasions that every single elf in the series is at least potentially bisexual. Although most of the characters seem to express a preference for one sex or the other there are several examples of elves in heterosexual relationships taking time out to have same-sex flings as well, perhaps the most notorious being Leetah and Nightfall's nude dance◊ in Volume 5 of the collected series.
Similar to the 51st century in Doctor Who, the eponymous superhero of Midnighter once tried to explain to a woman from the 95th century that he's gay. She didn't understand the word, and revealed that in the 95th century Everyone Is Bi (apparently that's enough of "everyone" that they no longer even have words for monosexuality).
In Alan Moore's League of Extraordinary Gentlemen the character of Orlando is definitely bisexual, but this makes sense because Orlando is an immortal who changes sex randomly and without warning. However, pretty much all of the female characters (be they historical or mythological) that Orlando spends any amount of time with are bi for Orlando. This includes Mina Murray, the heroine of the series, Fanny Hill, Venus (of course), Marguerite Blakeney and others. And when Orlando is a male, it's at least hinted at and also sometimes explicitly stated that many of his male companions are bi for him also.
In Century: 1910, Mina, angry with both Orlando (who's male at this point) and her partner Alan, announces that she'll be in self-Exile To The Couch, and they'll have the bed to themselves. She also comments that she likes Orlando better when (s)he's female.
In Artesia, all the Daradjan women seem to be bi, and the men at least have no problem double, triple, or even quadruple-teaming a willing woman. No male characters have yet been shown to be explicitly bi or even homosexual, though.
In Jaime Hernandez's "Locas" stories in Love and Rockets, practically every female character is bi, even the lesbian-identifying ones. Except for Vicki, who's a homophobe.
In C'hou in With Strings Attached there are no sexual taboos amongst the C'hovites; hence, Everyone Is Bi. For example, when Ma'ar asks George to sex with her, she tells him to invite the others in and they'll all enjoy one another. (Given that the four are firmly heterosexual, George agrees to sex but asks to leave the others out of it.) Grynun says that inside the castle walls, the four can have sex with each other but with none of the Idris until she has them first. And As'taris ogles the naked Paul and comments to Grunnel that if he'd known what he was missing, he wouldn't have treated Paul so indifferently.
This doesn't apply on the Hunter's world; he talks of the “shameful female-men.”
Choose two characters in all of fiction. There is a fic pairing those two. Now notice how gender was not specified in the prompt. That is because everyone can be bi in Fan Fiction.
To the extent that they have laws dictating that everyone must have at least enough heterosex to produce a couple of children. After that they can go back to their real loves.
In Piers Anthony's Apprentice Adept series, the serf class on Photon can be inferred to be mostly bisexual - mostly out of having to cater to the whims of the Citizen class (lead character Stile mentions "consensual heterosexual encounters", implying there's been homosexual ones and possibly voluntary). There are however, only two verified bi characters (Tania and Tsetse - naturally both female and hot.)
Note that this only became the case in the last book. Prior to that, homosexuality was barely even sidebarred.
In Merry Gentry, pretty much all of the Fey are supposed to be openly bisexual. Merry herself is all about the heterosex, of course, so that she can reflect the author's own feelings about sex with another woman, which seem to be "ewww gross but that's actually kind of interesting".
In Hamilton's other series Anita Blake: Vampire Hunter the number of minor and major male characters who aren't bisexual can probably be counted on one hand (and there are quite a few male characters) including just about every one of Anita's love interests. Interestingly it is made very clear more than once that Anita herself is not interested in women and most of the female characters that are tend to be pretty villainous.
In S.M. Stirling's Draka novels, the old 20th-Century genetically-unmodified-human Draka are straight, gay, or bi as the case may be (and with no shame or self-consciousness about their orientation ever); but the genetically-engineered New Race or Homo drakensis Draka, portrayed in Drakon, are all bisexual (and their sexual appetites always ravenous).
Anne Rice 's vampires are functionally asexual, but there's a lot of erotic subtext to their interactions nonetheless.
Arguable with some other characters in the series, but Lestat is absolutely bisexual; he has an intense love affair with his male best friend before becoming a vampire (nothing subtextual about it; they kiss multiple times, live and sleep together, and are strongly implied to have sex), and harbors deeply erotic thoughts about him even after the transformation. Considering the nature of his inclinations and relationships afterward, it's pretty obvious that he retains a pronounced attraction to men even after his ability or desire for physical sex is lost when he becomes a vampire.
In Armand's book, she finally had the main character have sex with persons of both genders before he was turned.
The same author's Cry To Heaven runs on this trope. The main (castrated) male character has love affairs with people of both genders, although his same-sex relationships are more numerous and generally more dwelt upon by the author. His two most lasting affairs are with another castrated man and a woman, respectively, and he thinks of both of them as the loves of his life at different points in the novel. He even carries on a sexual relationship with a cardinal in Rome, who (at first) justifies the affair with the church's belief that castration renders the person neither male nor female, and thus a loophole to rules about celibacy. It doesn't last.
Larry Niven's Known Space. According to one interpretation, which is in line with the way the Puppeteers themselves generally explain it to non-Puppeteers, Pierson's Puppeteers can be regarded as an entire race of bisexuals. Males are sapient and produce gametes, while females are non-sapient and provide gestation space. There are males who produce sperm and males who produce eggs, while females contribute no gametes but carry and bear the offspring. The female dies after childbirth, and the gay couple raises the kids, no sex except reproduction though. Would be Everyone Is Gay if the couple didn't have sex with a female at one point. An alternate interpretation, one that a human character stumbles upon and considers more accurate, is that the non-sapient "females" are in fact a different species that the Puppeteers are parasitic upon - similar to the Ichneumon wasps that lay their eggs in live insects so that the wasp larvae can eat the host alive. The "sperm-producing-male" is the Puppeteer male, while the "egg-producing male" is the Puppeteer female.
An astonishing number of the female characters in the Claudine stories are bi. This is shrugged off with the ignorant contemporary views of sexuality- as Claudine's husband says, "What you little animals do is charming and doesn't mean anything." It's another matter entirely if a man should be interested in both sexes.
In the novels by Gregory Maguire (especially Wicked), everyone is bisexual until they state they don't care for one or both genders. (Especially if you're descended from the Thropp line.)
Hell, in terms of ruining your childhood, it wouldn't be much of a stretch to pin this on the Baum Oz books.
In Fiona Patton's Tales of the Branion Realm, pretty much everyone is bi, and this goes completely unremarked. There is even a Guild of Companions (a cross between bodyguard, courtesan, same-sex sexual partner, and spy) who are contracted to the nobility. The first book revolves around the Crown Prince's relationship with his mother — she wants him to get married and have an heir, he would prefer to shack up with his Companion. He starts a civil war over this. His mother, BTW, has four Companions of her own.
In Robert A. Heinlein's Time Enough For Love, Ishtar and Galahad agree to "Seven Hours of Ecstasy," even though they have so far seen each other only in all-concealing biohazard-protection suits and, thus, neither knows the other's sex. It is strongly implied that in their society (the Howard Families colony-world of Secundus), it is considered in bad taste even to care.
Each one is pleasantly surprised to learn the sex of the other, as Ishtar is oddly tall and Galahad is oddly short. And both are absurdly beautiful, particularly Galahad (think the statue of David, except not terrified).
It's also implied in Stranger in a Strange Land that sexual bonding in the Church of All Worlds can occur between any water brothers regardless of gender, although this is done extremely subtly by Heinlein's standards.
In I Will Fear No Evil, Johann (a dying billionaire) has his brain transplanted into a new, young body and becomes Joan. With the body of a woman and the brain of a man, is his attraction towards women or men the gay one?
An Informed Ability in the Doctor Who Expanded Universe. In All-Consuming Fire Benny says that bisexuality is the norm in the 25th century (although she personally isn't). Subsequent books set in Benny's home era have offered no evidence of this.
In The Neanderthal Parallax, everyone in Neanderthal society is Bi, being expected to take a "man-mate" and a "woman-mate".
Word Of God about the Liaden Universe novels is that Liadens should be assumed to all be bi. We see several homosexual encounters but all the permanent arrangements we see on screen are heterosexual- unsurprising for a culture that puts huge weight on providing an heir.
Swordspoint and The Privilege of the Sword — perhaps not everyone is bi, but the only monosexual major characters are the villainous Lord Ferris (a depraved heterosexual, perhaps?) and Marcus, whose straightness may or may not have been caused by childhood trauma (thereby inverting Rape and Switch).
Due to the nature of a dragons mating flights and the fact that over 50% of the dragon population are females that mostly take male riders, it's implied that at least 80-90% of dragonriders in Anne McCaffrey's Dragonriders of Pern series are at least bisexual. Only Gold riders (1%) and Bronzeriders (5%) are exclusively heterosexual.
In Janet Morris' The High Couch of Silistra and sequels, while being straight seems to be the social norm, homosexual sex is apparently commonplace and completely acceptable. Estri, who begins the novel as a well-woman has a lot of experience servicing men, but when a customer essentially forces her into lesbian sex with another well-woman, she admits to being very aroused by the experience. Chayin and Sereth, two of her primary lovers, also have sex with each other, though only off-screen.
In The Culture, considering that the line between species and gender is completely blurred and deconstructed, everyone just seems to go with what they like, no matter what that is. And considering that Culture humanoid citizens can change gender and move into non-humanoid bodies with multiple sexes and... Look, suffice to say that if you brought up sexual orientation to a culture citizen they'd presume you were from a very very backwards planet.
This is reflected in their language, which does not differentiate between sexes.
In fact, Gurgeh from The Player of Games is considered bizarre because he's straight and has never changed his sex. A friend of his also seen as weird because she spends almost all of her time as female.
In The Stone Dance Of The Chameleon by Ricardo Pinto, sexuality is pretty much discarded as a point of interest, where straight and gay relationships are equally common-place. The problem comes where you have the Chosen (who are the undisputed 'supreme race' so to speak) and the other races (who act as their slaves and who aren't even allowed to look at their faces). Considering that all subservient races are completely oppressed by the Chosen and horrifically bound by The Law, their tends to be a lot of sexual abuse, rape, and subjugation of both genders of those races ... and some of the stuff the more sadistically-minded Chosen are into crosses the line of 'torture' and becomes Brain Bleach incarnate (Up to Eleven). In fact, when the main character (who is a Chosen, but brought up away from their culture), finds out that another Chosen has raped his half-slave brother, said Chosen can't understand why he's so upset and is genuinely shocked that he cares so much. Nuff said.
Arthur C. Clarke's novel Imperial Earth includes a line in which the protagonist's best friend is described as "aggressively normal," because he seems to have no preference between males or females — most people in the late 23rd Century go both ways but have at least a slight inclination toward one gender or the other.
Ursula K. Le Guin's story collection A Fisherman of the Inland Sea deals with attempts at instantaneous space travel. The final story, Another Story or a Fisherman of the Inland Sea, introduces the traditional culture of Planet O where pretty much everyone has to be bi for a successful marriage. Marriage is not between two people but four, two men and two women—a man and woman of the Morning moiety and an Evening man and woman. The Morning woman and Evening man, the Evening woman and Morning man, the Morning woman and Evening woman, and the Morning man and Evening man all have sex with each other, meaning every partner in the marriage has a husband and wife from the opposite moiety. (Sex within the moiety is forbidden, so the Morning man/woman and Evening man/woman in the marriage are strictly platonic. In fact, it's perfectly proper for siblings to be the Morning or Evening half of the foursome. Yes, this does mean that you are banging the same woman that your sister is regularly doing, and also the man who would otherwise be your brother-in-law.) The protagonist's mother, a Terran woman of Japanese descent, married in this way to be with the man she loves but finds it strange many years into the marriage, even though she is on good terms with her wife in the marriage.
In The Left Hand of Darkness the setting is a planet where the dominant species have no gender until they conceive.
In Palimpsest it's not as much as Everyone Is Bi but "Everyone is so desperate to go back into Palimpsest that they will slept with anybody with the mark, no matter the gender".
Many Mary Renault novels fit this trope. She wrote a lot of historical fiction set in ancient Greece, where bisexuality was considered the norm, for males at least. Some of her Greeks are exclusively or near-exclusively heterosexual or homosexual, but many aren't. The characters in her contemporary novels are also often quite flexible about gender.
In the Lyremouth Chronicles series by Jane Fletcher, this is the standard situation for the people on the mainland (the islanders have a more mediaeval approach towards same-sex relationships), to the point where being exclusively attracted to a single sex is almost incomprehensible and rather rare. The closest the mainlanders get to homo/heterosexuality is preferring a single sex, which is put on the same level as a preference for tall people, or people with blond(e) hair.
Most of the characters in Bret Easton Ellis' novels are bisexual. They are usually depicted as having sexual encounters of both kinds within the storylines. It may be a bit of Writer on Board.
In Outlander Leander by Eisah, Leander and Ellora argue which one will flirt with a Geuranian man to get information. Neither ever considers that the other man would prefer one gender or the other.
Dorne in A Song of Ice and Fire has this reputation as its hat, having a more more liberal take on homosexuality (and sexuality in general) than the rest of Westeros. However, while several Dornish nobles are openly bisexual, Martin's standard treatment of sweeping generalizations suggests this reputation is exaggerated.
In Torchwood, everyone in the main cast is at the very least bi-curious. The Doctor Who episode that introduces Jack Harkness states that his omnisexuality is common for the century (51st) that he grew up in, in which the Captain Kirkapproach to first contact has been the norm for so long that species and gender lines have become muddled. As Steven Moffat explains, it's a future where the whole human race is pansexual. Of the Torchwood gang, Tosh and Owen are properly bisexual, Ianto is either just bi for Jack or lying about not being into men who are not Jack, Suzie is seen snogging Gwen, and Gwen doesn't elaborate on which aspect of being kissed by various female villains weirds her out. (Word Of God has confirmed that Gwen is also bisexual).
A flashback involving two female members of Torchwood 3 in the 19th century (Alice and Emily) shows that they are a lesbian couple. At least one of them may be bi, given that she finds Jack to be pretty, but that may simply require the possession of sight.
You can probably just safely assume that everyone in a Russell T Davies show is bisexual unless explicitly stated otherwise. Davies has explicitly stated that he doesn't really believe in rigid definitions of sexuality, and that all self-identified straight people have at least minor gay tendencies and vice versa. Steven Moffat uses this trope a fair amount, too, though not as much as Davies.
The Daily Show. Everybody on the show has either shown bisexual tendencies on screen, or simply mentioned having had sex with both genders.
More often than not, the bisexuality is played as either comedic misunderstanding themed (Jason Jones once mentions that he married a gay man, after misinterpreting a recent ruling allowing gay marriage) or the correspondent is gay for pay (as seen with Rob Riggle, who had sex with men for money in order to earn the cash to buy an iPhone.)
J Michael Straczynski's philosophy for Babylon 5 was that in 2258, sexual orientation is a non-issue — not that everyone is bi, people just don't make a point of it. He suggested early on that one of the main characters was bisexual. This turned out to be Ivanova, but it only came up a couple of times.
Another interesting note is in the Babylon 5 movie River of Souls we find out that a holographic program was created with an image of Captain Lochley. Garibaldi discovers this program is especially popular with women.
It's never established whether Lochley's relationship with her late friend Zoe (temporarily resurrected in "Day of the Dead") was a sexual one, especially since they spent her one night alive just talking. On the other hand, it turns out that Lochley has missed her enough to be using "Zoe's Dead" as her computer password twenty years later.
In a subtler incidence, Marcus and Franklin pose as a newlywed gay couple when undercover on Mars. Nobody considers this unusual, though people aware that it's a cover do consider it funny - not the idea that they might be gay, but because they make such an ill-suited couple.
In Farscape, sexuality is completely by-passed as an issue. When you have trans-alien relationships, the actual gender of who you're attracted to becomes moot. It's also implied that humans are comparatively very rigid and backward when it comes to sexuality. Both D'Argo and Chiana seem surprised that humans generally start having sex so late (D'Argo first had sex when he was... seven, I think it was?), and Aeryn illustrates how in the Peacekeepers sex is fine as long as it doesn't involve any emotions. Chiana at least certainly doesn't make any distinction...
One particularly amusing episode featured an alien who, after being annoying comic relief for 47 minutes, closed out the show by coming on to D'Argo. Up to that point D'Argo and the viewers had assumed the alien was male (the role was filled by a male actor), but she assures everyone that she is in fact female and "Quite the looker." It's not clear whether D'Argo is more turned off by her masculine (by everyone else's standards) appearance or just by her personality. At episode's end Chrichton checks to make sure that Aeryn is in fact "The female of her species," (although he was probably just joking). It's hard to say where this episode puts the cast in terms of this trope, except to demonstrate just how weird and unpredictable ideas like sexual identity and gender roles become in a cross-species environment. If everyone isn't bi, they might as well be, because all terms and definitions are more or less out the window.
Chiana is quite obviously Anything That Moves but there are plenty of comments by/about D'Argo in particular that reveal he just might go both ways as well. The two that come immediately to mind are when Chiana proposes that Crichton have sex to get over his writer's block and when asked with whom, Chiana replies "Me, him, whoever", referring to D'Argo. Also, in the episode when John gets married and asks D'Argo to be his best man, D'Argo replies "I'm with Chiana now, John", the implication being that he'd be perfectly willing to be John's best man if he was single.
Nip/Tuck: Several of the characters have been bisexual and several of the heterosexual men have been raped by other men. Julia becomes a lesbian in season 4 but goes back to her usual two men Christian and later Sean. Liz a staunch lesbian relaxes her standards for Christian...and a male to transexual in the first season. Matt falls in love with Ava a transsexual, almost gets compelled to do gay porn to pay for his meth habit and briefly becomes a prison wife in the final season before reuniting with Ava. Merril Bobolit becomes Escobar Gallardo's prison bitch. Gina is seen to have had sex with several men and one woman in one episode. Kimber as a porn star frequently slept with women for work and frequently had threesomes to make Christian happy. Quentin Costa is a bisexual intersex rapist. Christian was molested by his foster father, raped by the Carver and briefly had gay dreams about his best friend/business partner Sean.
Exalted — This is common fanon, even though it's not exactly true. The setting has its share of bisexual characters (as well as straight and gay ones), but since so few mainstream tabletop games even mention sexuality, it can be jarring to see so many be confirmed to go both ways. The fact that sexual orientation is usually seen as no big deal in-universe probably doesn't help this perception.
Maid RPG includes seduction rules that are specifically mentioned as "not in any way governed by gender."
Pathfinder's producers are stated as having canonically asserted every iconic PC of the game is bisexual. Additionally, in the Jade Regent adventure path, there are actual rules for romancing each core NPC, and gender makes no difference. Apparently, everyone on Golarion is not only bisexual, but a perfect 3 on the Kinsey scale. This, combined with the fact that alignment, religion, and race are not factors, means that Rules as Written it's perfectly possible for a Chaotic Evil demon-worshiping android necromancer who is only going along because the adventure is promising a good, steady supply of corpses to animate to seduce and romance the Neutral Good Arthur analog who is dedicated to love, kindness, and beauty. Rule Zero would presumably stop anything too contrary to the theme of the campaign.
The Sims — For simplicity's sake, every Sim can romance any other Sim, at least as far as gender goes. There are some obstacles, but none of these are gender.
The Expansion PackThe Sims 2: Nightlife complicates things: positive romantic interactions increase a Sim's invisible attraction score to that gender, allowing better chemistry. Gender preference can also decay, so if your Romance Sim flirts with male Sims all the time, they can suddenly stop being attracted to female Sims. Pretty weird, even for a game like The Sims. Whereas if they flirt with both they will be attracted to both.
The issue with The Sims was that a Sim would flirt with everyone. They changed it in 2 so that while a Sim would be amenable to sex with either sex, they would only autonomously have one preferred sex, defaulting to heterosexual but homosexual if the user chose to play them that way.
In The Sims 2, Sims are asexual by default untill a romantic intereaction is performed by or on them.
The asari in Mass Effect are a One Gender Race of blue women who can breed with anyone (any species, any gender). Needless to say, many asari appear to be bisexual, because they seek people to mate with based on different criteria (with a focus on species, not gender).
World of Warcraft - One of the succubus's abilities, Seduce, can affect males and females equally.
Similarly, each version of the Theramore's Fall scenario has a male enemy that using a stun ability called "Dashing Smile", which affects players of either gender.
During the Valentine's day event, PCs can exchange pledges of adoration with NPCs for love tokens. Gender is only a factor in that female NPCs will romance PCs wearing cologne and male NPCs will romance PCs wearing perfume. Either gender can wear either cologne or perfume. Worth mentioning that to get the Largest prize you need at least twenty pledges from each city, Darnassus is all female and Ironforge is all male. On Horde-side, Silvermoon is all male.
In City of Villains, Succubi have a power called "Come Hither" that prevents players from being able to attack them directly. Interestingly, it works on both genders just as easily. The spell is broken immediately if the Succubus attacks the player.
Quite possibly the men of Metal Gear, where there are three canon bisexual characters — Colonel Volgin, Vamp, and Dolph himself. There's also subtext between Solid Snake and Otacon despite their own things for the ladies, and while Big Boss has no canonical partner he benefits from Everyone Being Gay For Big Boss with EVA/Big Mama and especially when Ocelot goes so far as to sacrifice his own personality/self by using nanomachines and hypnotherapy to make himself think that he was Liquid Snake — hence "Liquid Ocelot" — throughout the events of Metal Gear Solid 4... all for the cause, all for Big Boss.
In Metal Gear Online, both male and female characters can learn the Charm ability which causes them to do a sexy dance, stopping other characters in their tracks. The sexy dance does not discriminate based on gender.
Neither do the play-boy magazines
Big Boss can also go on a date with Miller at the end of Peace Walker and take photos of him on the beach in a thong.
That they have sex afterwards may also be relevant.
In the main Pokémon games, "Attract" only works on Pokémon of the opposite gender, but in Pokemon Mystery Dungeon Rescue Team Red and Blue, where only the player character has a definite gender, Attract works on (virtually) everyone.
While "Attract" affects only the opposite gender in the main series, the same cannot be said to "Charm" and "Sweet Kiss", though the earlier might apply to the Pokémon's cuteness.
The Elder Scrolls III Morrowind - if you get Personality stat high enough, NPCs of either gender, everywhere that you go, will greet you with the phrase "I like what I see". This is, of course, regardless of which gender the PC is. Which implies either all of them being bi or massive Stupid Sexy Flanders-style phenomenon with you.
In Fallen London, there are storylines that allow the player to romance partners of any and every gender...including at least two threesomes. In fact, the text is always written so the PC's gender never makes a difference; "the Dashing Smuggler slips his arm around your waist", for example. (It helps that the PC's gender can be set as "Gentleman", "Lady" or "There are people walking around with the faces of squid—squid—and you have the nerve to ask me my gender?".)
In Dragon Age II, all the main romance options are available to both genders (appropriately, one of them is voiced by Eve Myles from Torchwood). DLC character Sebastian is the only heterosexual-only romance, as well as being non-sexual.
Several of the non-romanceable companions are also implied to have bisexual tendencies to varying degrees. Aveline kisses Hawke regardless of gender at the end of her chapter 2 loyalty mission, Bethany expresses some curiosity about Isabela's sexual history with other women and Varric has some slightly flirty dialogue with Anders (though his heart belongs to Bianca, his crossbow). Even Bianca apparently feels some "confusion" when Isabela suggests she needs "a woman's touch on her trigger."
In Guild Wars 2, the Sylvari are noted to be like this. They're a race of plants mimicking the humanoid form, are incapable of sexual reproduction, and socially have no concept of gender. Developer blogs about Sylvari society and culture have noted that Sylvari consider gender completely irrelevant, including in matters of love.
In Dragon's Dogma, you can end up with anyone. Whether it's a male or female Arisen, you can have male on male and female on female. Some scenes don't even change regardless of gender, and this will cause some characters to come across as this.
In Borderlands 2, any character that shows some attraction to the Vault Hunter will do so regardless of the Vault Hunter's gender, several gay relationships are mentioned in passing, and virtually everyone in Sanctuary has some interest in Mad Moxxi.
In Space Colony, it is possible for any two colonists to become romantically involved.
Bisexuality is the norm for the drow in Drowtales, with purely heterosexual and homosexual people as minorities.
El Goonish Shive is a funny case, since characters who have been genderbent by Tedd's TF Gun develop a temporary attraction to the opposite gender. Depending on how you look at it, between one and two characters are naturally bisexual (Ellen is an Opposite-Sex Clone who has all the memories of her original male heterosexual self, and hasn't fully come to terms with the fact that she's attracted to guys as well as to girls. Grace is "Teddsexual" regardless of Tedd's current gender.), but seven of them have been genderbent, and of those the four straight ones have all had to deal with attraction to both genders. (Justin and Nanase are exempt from this due to being naturally gay—when genderbent, they just get even more attracted to guys and girls, respectively.)
While only two characters in Ghastly's Ghastly Comic have ever been called "bisexual", almost every cast member has slept with, kissed, or had a Stupid Sexy Flanders moment with members of both sexes at one point or another.
Concession: not everyone is bi, but a large proportion of the cast are. Joel, Matt, Roland, Angie, Emily, and Chelsie are all described as bisexual on their cast pages.Matt and Emily have only actually been seen interacting romantically with their respective same-gendered partners Joel and Kelly, but the others have all demonstrated at least some bisexual tendencies onscreen. Nicole (who is actually a male named Aaron), Cecil, and Kelly are gay. The other ten characters named on the cast page are heterosexual counting Kate, who is a practicing pedophile whose personal harem is made up of male preteens, plus the male-to-female Transsexual Chelsie, though she also slept with the adult Artie and her own twin brother - so, not everyone, but a much higher proportion than real life. A remarkably high proportion of gay and bi characters show up in the randomly generated background character population as well.
A few heterosexual characters have had homosexual encounters though, such as Artie's drunken foursome that included another guy.
And recently Rick let slip that he has had "hot gay sex" at least once, his girlfriend was not happy despite the fact that she sleeps with women fairly often.
Ménage à 3 starts out with a varied cast of gay and straight guys/girls with one Bi girl. By now, every single named character (and plenty of one-off extras) has had at least two or three instances where they've shown more than a little attraction for both genders, with the exception of the landlady and the guy who stays indoors and thinks it's still the 80's.
Homestuck: Alternian culture is binormative. Trolls reproduce rather like social insects and do not form families, so sex is pretty much cosmetic. Their emotional lives center around 4 types of romantic relationships, 2 of which are usually sexual. One troll (Karkat) even shows astonishment when the human characters try to explain the concept of monosexuality to him.
Much to the delight of the comic's sizable yuri and yaoi fanbases.
According to Word Of God, it is possible for a Troll to be monosexual, but they don't recognise it the same way we do: whereas heterosexual humans would deny advances from someone of the same sex because they are heterosexual, a heterosexual troll would do so because they don't find them attractive. It's more like a kink or fetish for either gender than anything else. According to Hussie, Kanaya is only sexually attracted to women although she's had romantic relationships with men.
Played with a bit later on. Dirk and Roxy both grew up isolated from other humans in a future world dominated by the troll queen's ideals. While they both decline to put labels on their sexuality, Dirk prefers to think in Alternian terms, but Roxy maintains that orientation IS a real phenomenon in human sexuality and has to be taken into consideration. This may be because Roxy has had an unrequited crush on Dirk and "he's gay" is a lot less disappointing than "he doesn't find me attractive enough even though I AM the last girl on Earth".
In Curvy everyone seems capable of making out and having sex with virtually anyone else, on a moment's notice.
Pretty much the entire cast of Magick Chicks has shown that they wouldn't turn down either sex, though Faith's abilities make some of them ambigous.
Gaia Online's Valentines 2009 event allowed the users to send Valentines to a number of NPCs, who would respond or refuse according to programmed turn-ons or turn-offs. Liam, Devin, and Vanessa refused Valentines based on the sex of the avatar who sent it to them; everyone else had qualifications based on other things and would respond regardless of gender.
In addition, there have been three Chance Item sets based on Dating Sims. The user can freely choose whether to pursue a male or female love interest in each, and none of the interests are restricted by the user's gender.
Drawn Together - Every cast member seems to have slept with or kissed at least one other character of the same sex.
Superjail. Maybe? It's not even entirely clear about the gender of the collective main cast. In any case, no one seems to be deterred in their crushes by little things like transsexuality or gender-switched counterparts from another planet/dimension or people who can apparently spontaneously give birth through their anuses. Or death.
Family Guy - Almost every, if not all characters has had bisexual experience on the show.
Given that in the common perception of western society there are still No Bisexuals, there's a good chance that a great number of creators who include bisexual characters in their works are bisexual themselves or at least have spend some thought on the issue. Once one is used to the idea that everyone might be bisexual and just haven't met people that are their type, there is nothing to stop one from declaring any character bisexual when it would fit the story. Which might explain the frequent ocurrance of the trope to some extend.
In a 2007 study, about 20% of the North American population admitted to being bisexual.
Scientist Alfred Kinsey's theories postulated that the great majority of the human race was bisexual in some shape or form. He saw orientation as a sliding scale, with pure heterosexuality (0) and homosexuality (6) as the extremes and no-preference bisexuality (3) as the center. Only a small percentage of the population are truly 0s or 6s, and 3s are also relatively rare. Most identify either as straight or gay but have some degree of attraction to both genders, even if only of the Stupid Sexy Flanders variety.
Recent studies measuring genital arousal tend to find non-zero arousal to all types of human sexual images. This is sometimes suggested to reflect situational embarrassment, but it could also have real implications for how our sexual responses work.
Animals who engage in sex for recreation or group bonding generally do so with both sexes. Bonobos and dolphins are both known for lots of quickies with anyone ready and willing. Note that these are both highly intelligent species with human-like social habits, suggesting that some degree of bisexuality is a natural result of the ability to enjoy sex and bond through it.
The sexual mores of the ancient Romans can seem like Blue and Orange Sexuality from a modern perspective. Like many pre-modern societies, the Romans had little concept of "sexual orientation" as a personal attribute rather than an attribute of a specific sex act. The "orientation" of an act was not defined solely by the genders of the partners but also depended on their Roman citizenship status and on who was penetrating whom. In general, same-sex relations were permissible—although often derided as decadent and "Greek", i.e., "foreign"—as long as it happened behind closed doors and the penetrated partner was a slave or a sex worker.
There were examples of Roman citizens who were openly on the receiving end, including the teenage emperor Elagabalus, but this was scandalous enough that he was assassinated by his own Praetorian guards in favor of his cousin Alexander Severus.
Gaius Julius Caesar's Berserk Button was people saying that he had been the submissive partner in a relationship with the King of Bitynia. Cicero summed it up, when Caesar was pleading for Roman support of the man:
Cicero: Enough of that. It is well known what you gave him...and what he gave you.
The idea that "you're not gay as long as you're on top" was fairly common in pre-modern cultures and still shows up in Western culture in certain contexts (such as the stereotypical version of Prison Rape).
Ancient Spartans were more or less required to be bisexual. A Spartan boy's first relationship was expected to be with an older man, who would act alternately as father figure and lover, and there were several laws regulating the nature of this relationship (such as "if a boy is courted by two men, and he chooses the richer, he has to pay a fine"). By contrast, a Spartiate would have an Arranged Marriage with a woman he didn't know and be expected to consummate it in secret (though later they would be allowed to live together as a family). Things are less clear on the female end, but several hymns relating to women and their yearning for "choral leaders" exist..
There have been some writings that tell of the women shaving their heads, so their soon-to-be-husbands can ease into the idea of a sexual relationship with a woman.
There's a joke that goes like this: "If you give people two choices in flavors, nobody will stick to one just because they're told to".
Freud said that humans have a predisposition toward bisexuality, but become monosexual through psychological development, while the bisexuality remains in a latent state. In other words, Everyone Is Bi.
Some people (notably, Dan Savage) have suggested that female sexuality is naturally more fluid and less amenable to clear labels than male sexuality. In other words, Every Woman is Bi.
This is complicated by different social norms and relationships among men and women, with different stigmas and things like "Girl on Girl Is Hot" getting in the way, and has been arguably disproven by recent studies. Not to mention that Dan Savage is neither a scientist nor bisexual, so he's commenting on a phenomenon that he's not anywhere close to being an expert on, and mostly from a very mean-spirited perspective.
Danni Ashe, adult model and businesswoman, frequently conducts interviews with other adult models on her website. In these interviews the question of sexuality inevitably comes up, and the answer is inevitably some variation of being bisexual. Like, nearly every single one. Whether or not they're being honest is up to the viewer...