A trope is a commonly-occurring literary convention. When it comes to bisexual characters and themes in m/m and f/f literature, these are frequently harmful and erasing. I've offered reasons why these are damaging and how to adjust for that.
Lest anyone get defensive, this isn't about policing your fictional characters or your writing. Write whatever the hell you want. Just don't get defensive if bisexual people really, really hate it and call you out on it. I'm not sure why so many writers think it's fine. We are rightfully angry when people of color, people with disabilities, women, and gay or lesbian people are portrayed poorly (and yes, I realize fully that a person can have intersections of any or all of those; I'm not making comparisons for the sake of politics, only for what devices are called out in books and other media). Heck, we don't like it when any people group, oppressed or not, is stereotyped. Why should bisexual-identified people have to put up with terrible representations in fiction, especially when the story could be told without them?
My biggest frustration has been in identifying how and why these are harmful and being met with resistance. Your temporary annoyance that I said that trope was bad is definitely not anywhere near as bad as the actual marginalization bisexual people face in real life. Not only that, this is fiction. That means it came from someone's head. I'm not policing actual people's identities or behaviors. I'm asking that we not perpetuate in literature the very things used against people in real life.
Here are some common ones:
1. No one is bisexual. In other words, every book where there is bi erasure ever. The word is never used. The possibility never exists. Sexuality is determined by "gay" behavior rather than identity. (Hint: NO. It doesn't work that way in real life, either.)
Why this is bad: Well, for starters, we exist. No, really! Even men! It's true. Also, it negates the existence of non-binary genders. Implied is that there are men and women, and man + woman = straight, man + man = gay, woman + woman = lesbian. Pretty limited and limiting.
How to fix it: If you have a character who has previously had a heterosexual relationship, consider the possibility of it having been meaningful. Get in the character's head. Is this person honestly only attracted to people of their own gender? If so, what were the reasons for their past relationships? If not, then your character is probably bisexual, and that doesn't have to be a big thing at all. Their Moment of Revelation can be as simple---or as complex---as if they were gay or lesbian. Dig under the surface to find out.
2. Everyone is bisexual. Everyone is bi! Typical in fantasy or science fiction, especially among alien species or mythical beings. This can be done well, but it can also be done poorly. (It's also a common assumption that it applies to humans in real life; topic for another day.)
Why it's bad: It erases us by making it not an actual identity. "Oh, you're bi? P'shaw. So's everyone else. That's not a thing." Yeah, it is. Besides, in most stories, nothing ever really demonstrates it anyway, except maybe a lot of random threesomes (or a lot of random sex, period). Characters just go about their lives as though this is Genuine Fact, and there's never any examination of the realities of being out and bisexual or the differences that a fully bisexual species might have from humans. It's also often done to be "edgy." Trust me, this is not so. I mean, I don't honestly feel like I'm an edgy person, so I don't appreciate being treated like my identity is exotic.
How to fix it: Probably just don't do it at all unless you can do it with some nuance and show how that culture or society would be different from our own. Also, I suggest not fetishizing us.
3. Gay for you. This is when a character has never been interested in a same-sex partner but all of a sudden falls for someone of the same sex. They usually otherwise show no interest in anyone of the same sex, not even to notice a celebrity is attractive. They spend no time whatsoever thinking about what their hot-for-love-interest means and typically announce to the world (repeatedly) that they are now gay/lesbian.
Why it's bad: It's another form of erasure. This trope can be done well if it isn't done as a way of ignoring an entire identity, but it's typically in the same category as "gay" behavior means "gay." Falling for a best friend? Absolutely---can be so wonderful to read, just not when it implies there's nothing other than gay or straight.
How to fix it: Don't even make it a gay vs. straight vs. bi thing. This is one time when I think not mentioning any specific orientation works really well. It's about the feelings more than anything else. As a person who myself doesn't prefer labels outside of political/social context, I would like to see "I'm not sure how I identify, but I sure do know I like you!" in this case.
4. Gay "Bisexual." I hate, hate, hate this one. Teenagers will sometimes self-identify as bisexual because the world is geared toward heteronormativity and the expectation is grow up, get married, make babies, retire to a villa in Miami. I didn't even know the word "bisexual" until I was about 16. Eventually, those kids may identify as gay (some do; some don't and continue to identify as bi). This is not a thing adults typically do. This trope gets extra rants because usually, the character is pressured into identifying as gay as soon as they show even slight interest in people of the same sex.
Why it's bad: It perpetuates the myth that bi-identified people mostly go on to be "gay" or "straight." This is not true. Most gay and lesbian people have never identified as bisexual. Yes, some have, but not the majority, and there are just as many gay- or lesbian-identified people who realize they are actually bi. It's really, really important to believe people when they say they're bi no matter what. A lot of bisexual people, especially guys, get talked into identifying as gay because so many people think bisexuals don't exist. This is awful. We need to practice believing people when they tell us how they identify. Even if someone says, "I'm bisexual!" for five minutes before identifying as something else, believe them. They know best. No one ever has the right to label or pressure another person. No one. Not in real life and not in fiction. If someone identifies as bisexual, you the character or you the real-life friend DO. NOT. GET. TO. SAY. OTHERWISE.
How to fix it: Have bisexual-identified people be bisexual. Try to avoid having gay or lesbian people first self-label as bi. Even if it's "realistic," it's still a harmful stereotype. It's rarely necessary for plot.
5. Behavior equals orientation. A character has a non-heterosexual encounter. Now they're Officially Gay. Very similar to Gay for You but often isn't attached to one specific other person. It's often used in situations where adultery is the main relational obstacle but isn't enacted by the main character. For example, "straight" best friend messes with the main character's love interest and voila! "straight" best friend is now gay.
Why it's bad: I said it above. Those aren't equivalent. A person is not gay because they have a same-sex encounter or relationship. A person is not straight because they have an opposite-sex encounter or relationship. Similarly, not having a relationship doesn't negate someone's orientation. Oh, and don't even get me started on how those assumptions erase anyone who isn't gender-binary. (Of course, non-binary people typically don't exist in books either, but that's a rant for another day.)
How to fix it: If it's not the main character's journey, just don't do it. None of us like reading about ourselves as nothing but our sex habits. If it's the main character, don't pull a "suddenly gay/gay for you." Use the same fixes as above.
6. "Now I know why I never liked...": Any and all previous relationships are now examined and found to have "always" been lacking, although that's never shown within the text---only in retrospect. No possibility exists that any of them were meaningful in any way, especially not sexually. This is established by the author telling the reader or doing an information-dump explaining how bad it obviously had to have been.
Why it's bad: Not only is this a horrible way to erase bisexuals, it's also a terrible way to represent a person's former love interests. In the case of info-dumping, it's lazy writing to boot.
How to fix it: Don't detail specific past loves if they don't feature into the story. Don't create former boyfriends/girlfriends just to tell readers how "bad" it was. It's not necessary to work that hard. Readers don't need to know that stuff and are generally smart enough to get it without having it spelled out.
7. "No, really, I'm GAAAAAY!": This happens when a character established as gay enters a meaningful, loving, sexually fulfilling opposite-sex relationship. Instead of examining why that might be, the character just asserts their gayness more forcefully and discounts the relationship as some sort of accident. Nine times out of ten, it's done to provide a way for a "straight" character to become involved in a same-sex relationship after the person's partner is out of the picture.
Why it's bad: It's an ass-backwards attempt at not doing the above tropes. It would probably suck to be the partner in that relationship, honestly. It sometimes reads as a weird attempt to "normalize" same-sex relationships in a way that negates every other possible combination. That's pretty hurtful, and it implies that bisexual people with opposite-sex partners are in a "straight" relationship (nope). Oh, and did I mention it erases non-binary people?
How to fix it: Please just don't do this. Let relationships be what they are, and if you plan to have a character clearly in love and sexually enjoying people of multiple genders, call it what it is---bisexuality/pansexuality/sapiosexuality.
8. "I'm bi, but same-sex sex is the best thing ever!": Well, it might be, if your opposite-sex partner was the only person you were ever with and they were a raging jerk or super selfish in bed or whatever. It might be amazing simply because it's the first time being with a same-sex partner (but let's not forget the potential for awkwardness in that situation, too). More often than not, though, it's used to make it clear that it's inherently superior for the sake of keeping two characters in a relationship.
Why it's bad: It makes it seem like those of us who are bisexual are very, very strange. It paints opposite-sex relationships as so inferior that how could a person possibly want one once they've experienced the awesomeness of a same-sex encounter---that once you've had a taste of how awesome same-sex coupling is, you'll never want to go back to that lousy het crap again. It reduces relationships to sex and parts rather than whole people. All by itself, that erases trans people because it's so fixated on binary bodies/identities.
How to fix it: Just don't ever have your character say it. Let the sex speak for itself. There's no need for a character to say or even think "This is way better than when I had to suffer through het sex" or any other version of that sentiment. It's not necessary, and if you're relying on it...well, then, your sex scenes aren't very good.
I kind of get it. These tropes exist because a lot of people who aren't themselves bi have no idea what to do with us. Here's a thought: If you can write characters who are a different gender, race, age, nationality, or sexual orientation from you, then you can write someone bisexual even if you are not. It can be done! Many of us are happy to answer questions about what it's like. We're happy to look at your work and make sure you haven't done something hurtful to our community.
I'm not expecting bisexual characters in every story. I just want authors to stop acting like we don't exist and using horrible methods to ensure they don't have to write about us. Heck, I don't even really mind some of the tropes. It's just that they are typically used as ways to ensure two characters end up together under the assumption that it couldn't possibly happen otherwise. If you're tempted to write a bi-erasing trope to get your main characters in bed or in a relationship, rethink it because you are missing an opportunity to smash some stereotypes and mess with societal expectations. _____ For more excellent tropes, check out http://tvtropes.org/pmwiki/pmwiki.php/Main/HomePage. Sadly, nearly all tropes pertaining to marginalized people are negative (whereas other tropes can be positive or used in positive ways). Please try to avoid them.