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In Real Life, this phenomenon is occasionally called Bisexual Erasure. See also Suddenly Sexuality and A Threesome Is Hot. This trope is, of course, not Truth in Television in the sense that bisexuals exist. It Truth in Television in the sense that bisexuals have often been faced with disbelief from straight and gay people alike on the grounds they "have to be" one or the other. There is also the growing body of research showing that homosexuality is likely to have a biological cause, and attempts to "cure" it with psychotherapy or other non-medical techniques are not only usually ineffective but can often be dangerous, and the rejection of this trope by all major medical and psychological organizations such as the American Psychiatric Association. It is now illegal to subject minors to this in California, with other states following suit; additionally, conversion therapy has been found to be a violation of state consumer-protection laws (essentially, false advertising/fraud, or in trope terms, it's pure snake oil) in New Jersey (following a 2015 verdict), which likely portends similar findings elsewhere in the US at least (most states' consumer protection laws are worded very similarly).
The aspect of works of fiction where there is a lack of bisexual characters despite the definite existence of bisexual people In Real Life (1.8% of US population), and often don't even acknowledge their existence.
If the existence of bisexuals is acknowledged, they're usually depicted as very promiscuous and totally defined by sex, and sometimes even as predatory (mirroring prejudices leveled against real life bisexuals).
A few characters do manage to use a version of Ambiguously Gay to avoid scrutiny (see Hide Your Lesbians).
However, such characters will rarely call themselves bisexual; they may insist they're straight, or just leave it up in the air.
Meanwhile, since male characters often act as the Audience Surrogate, their sexuality has to be strictly heterosexual, lest the show be forced into the Minority Show Ghetto.