They look exactly like hot women, and they make out with other hot women. But they're actually aliens/immortals/bodysnatchers/etc. So it's not really gay, right?
Discount Lesbians are canon homosexual characters and couples who are either not human to begin with, come from alternate realities, have been magically or technologically altered in ways that affect their sexuality, etc., so that they don't really "count". This usually seems to be a device used within the story to maintain a status quo that lets the (presumed) audience watch two hot chicks making out without having to think about the associated real-world issues of homosexuality. On the other hand, the trope may come into play if the writer wishes to include a lesbian relationship in the story but is worried about Moral Guardians, thus becoming a way for a Rule Abiding Rebel to make an attempt at Getting Crap Past the Radar.
Note that they're Discount Lesbians only if they're included in the story in a way that obviates the need to address the associated issues of sexuality. This is usually done through othering homosexuality by associating it only with characters who have conspicuously different traits from the viewers. Characters used to draw attention to those issues are inversions, although this can still fall into Fantastic Aesop or Space Whale Aesop. If they're stated to be homosexual they're not Discount Lesbians, just a non-human who happens to be gay.
Basically gays... but cheaper. Commonly carries Unfortunate Implications (see page quote). It can be considered a subtrope of But Not Too Gay.
Compare Have You Tried Not Being a Monster?, Lesbian Vampire, Bury Your Gays, and (occasionally) Elfeminate. Related to Deceptively Human Robots because both are only as robotic/gay, respectively, as is convenient for the show. Also see Space Jews for aliens standing in for minorities. Related to Hide Your Lesbians in that both are a way of including apparently gay content without admitting to it. Not to be confused with transgender and metaphorically transgender characters.
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The current Kerrang! bumpers in the UK show a young Asian woman who flashes with yellow energy making out with a pink, suited androgynous humanoid with a mask, clearly intended to invoke the Pink and Yellow Rangers - but as only Yellow is clearly a human woman and Pink shows no gendered characteristics beyond its colour, it maintains plausible lesbian deniability.
Anime and Manga
This trope is frequently invoked by some Neon Genesis Evangelion fans whenever anyone mentions the Ho Yay between Shinji and Kaworu, what with Kaworu being half-Angel.
technically his body is human, he just bears the soul of Adam.
The entire premise of Simoun, where everyone is born female, only choosing a permanent gender at the age of 17. This automatically puts all teenage characters into the Schoolgirl Lesbians zone. However, by the end every possible gender combination is represented, averting the main reason for this trope.
Used for the women in Vandread, because each gender had essentially become a different race. The male equivalent is never revealed. The Majerans (women) form romantic bonds and two-parent families for their daughters. As for the men of Tarak, in the first episode, one of the background conversations during a military graduation has one guy asking another if they want to make a kid. The guy's answer was basically, "Sure, why not?", as if he had been asked to form a World of Warcraft guild. A later episode one guy is shocked the baby grows inside the woman as all men are test tube grown.
Kashimashi: Girl Meets Girl has the main lesbian character be a transformed boy, although it's heavily implied that s/he was a trans woman, but didn't know about his/her condition. Not even the voice-actor is changed with her gender!
The Cthulhu in Iczer One play this straight, being an one-gendered female-looking alien species. The titular Iczer-1 and Nagisa may count as well, with Iczer-1 being an alien android who seems to love Nagisa.
Alien from the Darkness has Flair, who spends most of the story seducing the all-female crew of the Zogne. While she qualifies due to being possessed by a tentacle-raping alien in search of breeding stock, the sex scenes Flair isn't involved in prove that the hentai has no problem with actual lesbians either.
During Joss Whedon's run on Runaways, lesbian character Karolina has a long-standing relationship with a shapeshifting alien (twofer!) who appears female entirely to keep her happy. Karolina herself is also an alien. (An alien who believed she was human, and was physically human, until her early teens, but an alien nevertheless.) This is an especially glaring example, because Xavin (the shapeshifter) has mentioned that members of his/her species don't stay the same sex all the time anyway (s/he compared changing sex to dyeing one's hair) and had a subplot where Molly, another teammate, was bothered by the fact that Xavin was only a girl sometimes (Molly eventually learned to accept Xavin as is). Karolina, however, insists that Xavin is female and only female, despite that Xavin does spend a lot of time male, too (and not just when it's more practical, either).
Shatterstar fits this, depending on which origin story you believe.
In Wonder Woman comics, the Amazons of Paradise Island often have romantic relationships with each other.
"We don't call it 'Paradise Island' for nothing."
The Neil Gaiman story "Murder Mysteries" actually has discount gay men. The characters in the main part of the story are angels, sexless and genderless, though they do generally take the form of human males. The angels who discover love end up as lovers, which means while there isn't any distinctive action, there's a lot of cuddling.
Ursula K. Leguin'sThe Left Hand of Darkness, dealing with a planet of people who are asexually hermaphroditic until they enter heat, at which point they can assume either set of genitals, is sometimes confused for this. It's not, for two reasons: first, the issues raised by this setup are the entire point of the novel, so there's nothing discounted about it; and second, the characters are not presented as substitutes for gays.
Discount Bisexuals? Discount Bisexual Threesome? In the Dark Nest Trilogy, Jaina Solo and one of her love interests, Zekk, were both snapped up by the psychic alien menace-of-the-week; they became Joiners, on the aliens' side and telepathically very close to each other. At one point they started fighting against Jagged Fel, Jaina's other big love interest. Jaina-and-Zekk had thoughts along the lines of "We should make him a Joiner and be Jaina-and-Zekk-and-Jagged! Yes, he is pretty. He would make the mating rituals much better."
It was also rife with Unfortunate Implications given that Soren was played by a woman. You could read it as "Riker's dick saves girl from lesbian colony: unfortunately psycho lesbians brainwash her back." Jonathan Frakes even suggested the female-identified alien be played by a male actor. Given that it was the early nineties though, that is pretty Fair for Its Day, as most shows would not even approach the issue.
The evil sexually-manipulative bodysnatching alien Mary in the Torchwood episode "Greeks Bearing Gifts" is a glaring example. Toshiko is at least on the borderline, given that Tosh is never seen to have a sexual interest in a woman before or afterwards and it's not made precisely clear whether her sexual relationship with Mary was bisexuality or mind-control.
A straighter example is Gwen making out with a (female possessed by a) sex-gas alien in "Day One".
Tina Greer of Smallville is a shapeshifting Psycho Lesbian obsessed with Lana. However, her every romantic interaction with Lana is while she's in the form of a male. She invokes it because she knows Lana is straight.
In Doctor Who, although there's not really a shortage of more humanoid queer characters, the fact Madame Vastra is a Silurian seems to be used to vaguely explain why the homophobia of Victorian Britain doesn't really figure for her and her wife Jenny.
In Alphas, Nina mind-controls the straight (and also uncomfortable with physical contact) Rachel into kissing another woman in a club, to cause a distraction and also just For the Evulz.
Done a lot by William Shakespeare because of his fondness of crossdressing plots. Therefore, when (for example) Duke Orsino falls in love with the handsome Cesario in Twelfth Night, it's okay because Cesario is really the very female Viola in disguise.
A pair of Tasen from Iji are in a relationship. This one makes a bit more sense in that almost all Tasen soldiers shown on-screen except for the Bonus Boss are female.
Finally averted in Mass Effect 3, where two gay characters are introduced to the crew and one comes out as bisexual. Conversations with these characters and others suggest that sexuality is almost a non-issue by that time.
Joshua in Another Day from The World Ends with You. This looks to be played straight until you remember that Joshua, as the Composer / an Angel, could be whatever sexuality or gender he damn well pleases.
Fallout 3 has exactly one lesbian couple in the game, and they're both ghouls (hideously deformed, all-but-ageless zombie lookalikes).
Guild Wars 2 has the sylvari, plant people who don't reproduce sexually, and to whom gender is completely irrelevant.
The vampire-ish Mystics in SaGa Frontier have a natural "Charm" ability that can attract others, none moreso that Orlouge, ruler of the Mystics. He gives a blood transfusion to a human girl named Asellus, one of the protagonists, and she becomes a half-Mystic in turn. Because of his magical blood, Asellus is more or less a lesbian (or vaguely bisexual, and this same blood causes a human girl named Gina to fall in love with her. She and Asellus have a sort-of-there Mayfly-December Romance should Asellus choose to remain a half-Mystic in one ending, they become a couple (with Asellus toting an all-girl harem to boot) in the full Mystic ending, and in the ending where Asellus becomes human again, she marries a man.
The Dragon Doctors features a town where all the men have been transformed into women. Their romantic relationships have generally continued, the town having become a huge market for Magitek that alters one's sexual preference to "bi." (The author has mentioned that the "orientation adapter" magic only sets you to bi, basically widening the spectrum of what you could be attracted to without otherwise altering what you're already attracted to, so it's not used as a cure for gayness or anything).
Tycho: I'd love to know why you have a rule for that.
In Homestuck, the trolls have a system of romance and mating that is much different from ours, and gender isn't really a factor in any of it. Monosexuality as we know it does exist, but is so rare that when John mentions the concept to Karkat he is thoroughly confused that it's even a thing in human society. According to Word of God, preference for a gender in troll society is analogous to a fetish insofar as it is an exacting preference rather than a preference swapped from the norm. It is not, however, analogous to a fetish in that it is something that the troll can ignore if they have it bad enough for the gender they normally don't prefer; Kanaya, for example, is only attracted to women as a rule. Unfortunate Implications are removed however with the explicit presence of gay and bisexual humans.
Variation: In 1/0, Terra the earthworm is introduced as a lesbian because Tailsteak wanted to include a female character who wasn't a viable romantic option for the guys. This is clarified by saying that earthworms are hermaphrodites, but he's choosing to identify them all as female, so they're all "lesbians". The fact that she's not a "real" lesbian is the key to her being with the man she loves without rejecting the author.
The Futurama movie The Beast With A Billion Backs contains a transdimensional planet-sized tentacle monster named Yivo. Shklee has a male voice, but everyone in the universe dating shklim isn't treated like an issue by anyone, at least not from the perspective of sexual orientation. Fry clearly states that Yivo loves both male and female equally and has no gender, and schlee even has schlis own pronouns.