Tenchi Muyo!: The discovery in the OVA that Juraians practice polygamy implies that Tenchi could do this. In the original series, he tends towards No Romantic Resolution by remaining friendly with all of the girls but not favoring any of them, but in the core continuity's tie-in doujinshi, it is clearly stated that he did eventually marry them all. This trope was originally called the "Tenchi Solution".
Both the OVA and GXP justify this by stating that the marriages are political: Ayeka and Sasami are granddaughters to Seto, the most powerful noble in Jurai who bosses the King and Sasami herself is gradually merging with Tsunami, the patron goddess of Jurai and one of three known 11-dimensional entities named the Choushin (literally "super-deity"), into a singular being; Noike is the adopted daughter of Seto and the female half of Kagato, which also means that she is a Naja Akara clone and as result has the same genetic makeup as Seto-no-onihime; Mihoshi is a member of the Kuramitsu family, one of the most powerful families in the universe; Washu is a Choushin and sister to the aforementioned patron deity of the Juraians as well as Mihoshi's ancestor; Tokimi eventually decides to live in Tenchi's house with her sisters, though she apparently doesn't have a romantic relation with him; Ryoko is technically Washu's daughter and therefore technically half-choushin; and Ryo-ohki is Ryoko's ship. As the third OVA and GXP quickly point out, Tenchi's household is the single highest concentration of power, Physical, Political, and Supernatural in the universe.
In the canon spinoff Isekai No Seikishi Monogatari, Tenchi's brother Kenshi mentions all of Tenchi's girls as his "elder sisters", implying that 15 years after the OVA end, they are still arround him.
As seen in the page picutre, in Tenchi Muyo GXP Tenchi's friend Seina, in a similar predicament with nine women (five main, four minor), has his problem solved for him by simply ordering him to "politically" marry his main five love interests. (The other four kidnap him just before the wedding and ravish him to get in on it; it apparently works out somehow) Tenchi even attended the wedding.
Dual! Parallel Trouble Adventure: At the end of episode 14, Kazuki is made to go alone into space, with both Mizukis, Yayoi, and D (all the girls in love with him) as his official staff, at the actual instigation of the all the major parental authority figures (the Sanada and Rara parents) in the series.
No big surprise, since it's from the same company that made Tenchi Muyo, and is in fact canonically considered to be connected to Tenchi Muyo! by The Multiverse concept.
In Happy Lesson, all 5 of the hero's "mothers" decide that he's outgrown the need for a parental figure, so obviously what he needs next is wives.
The manga version of Magic Knight Rayearth, with Hikaru declaring she's going to marry both of her love interests. Though what Hikaru actually says is that she wants to be with the ones she loves forever (Which is how Eagle describes marriage), and then defines the ones she loves to be virtually entire cast.
Mahou Sensei Negima! gives us "The Saishoudoukin/PolygamyImagine Spot incident", courtesy of Nodokanote the last person in the cast that is expected to be a Covert Pervert. Initially, she began to worry about her best friend's crush on Negi, but gets even more embarrassed when the first thing her magic book (which she uses to read her own mind) subconsciously suggests is a threesome, where a man has his wife and mistress in the same house. Or, taken literally, in the same bed.
What makes it even funnier is the fact that the mind reading is accompanied by a little doodle of her, Negi, and Yue sharing a bed. Ye gods, girl.
Said doodle was actually pretty innocent. The image her imagination cooked up a second later was not.
This is also the resolution to the Final Movie; put on the spot to pick one girl for a full contract, he goes ahead and forms full contracts with all thirty-one students. Even though you can't do that. Somewhere over the door of the Springfield family ranch (which will need expansion) is a sign saying "The Laws Of Magic Can Get Bent."
Code Geass teases at this one in the second season. Kaguya, devoted fangirl and would-be wife of Zero/Lelouch, is perfectly okay with his supposedly eyeing other women like his bodyguard Kallen or his partner C.C., because as Kaguya puts it, "Heroes are known for loving many women", and comments that the three of them are united in their devotion to Zero. Not that he's been eyeing any woman at all, really, but Kaguya does like to play The Ditz now and then...
She's not completely wrong, either, though. Lelouch himself may be a Celibate Hero, but C.C. has been sleeping in his bed from the beginning, and is the only one he trusts implicitly, and Kallen is only behind because of her own racial identity issues and questions of Lelouch's morality. Lelouch's attempt to wriggle out of her marriage proposal was pretty much accepted by every viewer, as well as Kaguya, to mean "I belong to C.C.". In addition, the three of them disappear at the end of the Black Rebellion, and reappear together. That Kallen is the only one of the three who socializes with anybody other than each other, and their free access to Lelouch's room - which is apparently also C.C.'s room - is the final nail in the idea. There's very few other conclusions.
Her claim of them being "The Three Court Ladies" is so popular with the fandom that it's practically a meme.
And then if you add in the fact that his father had 108 wives (being Emperor) and that Lelouch eventually becomes emperor as well... the possibility of this multiplies. Extremely quickly. Except, of course, that dear Lulu dies.
In one of the sound episodes Shirley, while thinking about how to give Lelouch her concert tickets, keeps imagining how he will reject her. In her Imagine Spot she starts pairing him up with various people, and it all ends with Kallen, Milly, Nunnaly, Rivalz, Suzaku and Arthur deciding to share Lelouch between them and inviting Shirley to join in.
There are also hints that Milly is interested in Lelouch... which, combined with her supporting Shirley's desire to date Lelouch, makes you wonder if she gave up on him, only started feeling that way in season 2, or was planning to seduce Shirley and become a trio. She is the woman described as "like a dirty old man" by her best friend.
In a later episode of the Sorcerer Hunters anime, Tira and Chocolat have a heartfelt sisterly talk about their feelings for Carrot, and it's strongly hinted that not only don't they mind sharing, but Carrot wouldn't mind being shared (OT3 for the win!).
The manga version handles the triangle differently; Carrot and Tira become the Official Couple at the end, and Chocolat decides to wish them well... as long as she's the first to have "Darling"'s child. The revival manga shows that she's accomplished her mission, with her as the mother of two children that she swears she can't remember the father of...
In the Maburaho anime, Kazuki uses his magic to create ten clones of himself, just so there's enough of him for the girls to share. This solves nothing whatsoever, as the Clingy Jealous Girl wants them ALL for herself.
In the Mega Man Star Force anime, Luna and Harp Note at one point try to make MegaMan choose between them. We don't see exactly what Geo says, but he goes for this solution.
In the original Japanese anime at least, Subaru (Geo) says "We're all just friends, right?" with which the girls are seemingly satisfied.
Can happen in the third game of the series as well, if the player wants. The player can either have Geo save Luna's things (and have an extra scene with her), Sonia's (and have an extra scene with her), or... Bud's, in which case there is no extra scene with the girls.
Kanokon has an odd variation in which the guy in question doesn't even make the call; the last episode has Chizuru and Nozomu agreeing to share Kouta.
Also, whenever he tries to pick and just go for it, he is interrupted.
Umi no Misaki, by the author of Ai Yori Aoshi. A Tokyo teen moves to the southern island his late mother grew up on, and finds out that not only is he the prophesied reincarnated Dragon God of the local religious offshoot, the three hot girls that have been treating him badly are shrine maidens dedicated to serving him with their minds and bodies. Oh, and the Dragon God is supposed to give one (at least, it has not been clearly stated) his "favor". Of course, now the three maidens have fallen in love with him and trying their (non-violent) best to get the Dragon God to pick them, while the guy is attracted to and likes the three maidens as well . The story is working itself into a OT 4 ending, since this is really not a dark series, and having the guy pick only one girl would probably tear the other two to pieces.
Now confirmed. Despite first the Rising Dragon Festival and then Shizukus attempted suicide trying to railroad him into a choice, Nagi refuses to lose anyone and essentially forces the island and the Dragon God to accept that he will be with all three of them forever. The epilogue chapters show that the four of them are getting along perfectly well despite the oddity of their relationship.
In Ai Kora, Hachibei explicitly says he plans to do this. Though this becomes especially implausible after Tsubame-sensei gets a new boyfriend, and Kirino moves away after becoming an idol singer. He seems to have abandoned it completely in the final volume after realizing how much he loves Sakurako.
In Saber Marionette J, Otaru is shown to end up with all three of the girls — Lime, Cherry, and Bloodberry.
Subverted in the second season, Saber Marionette J to X, in that Lime, Cherry, and Bloodberry end up getting turned into babies, and Otaru raises them as his daughters.
As seen in chapter 162 of To Love-Ru, Lala not only does not mind the possibility of both her and Haruna marrying Rito but is actively enthusiastic about it. Later, he accidentally says "I love you" to several of the other girls (even thought it was meant for Haruna, and Lala does not mind sharing him with them too. And it's implied that, by Deviluke law, he could take even more wives once he becomes king after officially marrying Lala. Hooray for the harem?
In the sequel To Love-Ru Darkness, Momo explicitly wants this outcome (with her in the harem of course) and knows that the biggest hurdle is Rito himself.
In Sekirei, the standard trope of having to choose one girl is actually part of the Big Bad's plan. Minato considers this to be a cruel choice, and declares his intention to be with all six of his Sekirei forever. Seo and the twins seem to have gone with this option, as have Sanada and his Power Trio.
Despite the implications of the opening quote from Hayate the Combat Butler, it seems that he's probably going to take the Dump Them All route. Or at least take the next 40 years to clear his debt before he tries to choose anyone. Doubtful that any of the girls would be willing to wait until they're 50 for a conclusion.
Except for his true love (whoever that is)! So actually, its a pretty good plan.
Recent events in the manga make this...complicated. For starters, the debt issue is kinda-sorta handled by Nagi giving up her right to the inheritence to help Hayate save Athena. Hayate will likely be Nagi's butler forever simply because she means a lot to him and he wants to help her. In addition, another issue is solved in the way of Hayate getting over his guilt about his past with Athena so he may be open for the possibility of romance (at least more-so than he was). Time will tell exactly how this gets resolved but Nishizawa has already proposed that both she and Hinagiku marry him so the option is still technically on the table.
The idea of Marry Them All was actually raised in a recent Rosario + Vampire chapter. Pretty much all of Tsukune's harem except Moka were almost immediately in favor of the idea.
Of course they did and of course she wasn't. Moka actually stands the best chance to "win" all on her own, having Tsukune completely to herself (well, if you don't count her split personality) and leaving the others out. None of the other girls have much of a shot at winning individually, but as a harem, they can avoid losing entirely. Yukari deserves a special mention, as she was already aiming to achieve a One True Threesome with herself, Moka and Tsukune — a harem solution gets her that, even if she has to tolerate sharing with Kurumu and Mizore (and possibly Ruby).
Complicating things further is that the one he has already chosen isn't the real Moka — it's technically a personality constructed by the rosary itself. So the rosary is a haremette. (Though OTOH, Outer Moka is somewhat of a reluctant Shipper on Deck for Tsukune/her other self, and also Tsukune is warming up to Inner Moka too.
In Bleach, Orihime suggests something close to this effect. Though her feelings for Ichigo are obvious from the beginning of the manga, she eventually asks Rukia point-blank if she has feelings for Ichigo. Rukia denies this, and Orihime is actually disappointed because she feels that if they both loved Ichigo, they could gang up on him and the "girls would win".
In the 23rd anime ending, Shunsui is shown having a date with two girls: his lieutenant Nanao Ise and his ex-lieutenantLisa Yadomaru from the Vaizards.
Given the latter's taste in "reading materials", this is entirely plausible. How they convinced Nanao to go along with however...
Though it should be noticed that, in the Visual Novel, this can be played straight. Not only there's the possibility of having Makoto, Kotonoha and Sekai engage in a three-way sex scene (with bonus Les Yay snowballing between the girls), but there is at least one ending in which Kotonoha and Sekai DO reach an agreement and decide to "share" Makoto's affections. (It IS rather hard to get, however.)
In SHUFFLE, particularly the original H-Game/Visual Novel, no matter who Rin ends up with, Shin-ou reminds everyone that polygamy is legal in Shinkai.
In the sequel/expansion Shuffle! Essence+ this is explicitly the ending for Kareha that Rin agree to marry both her and her sister Tsubomi and more specifically in Tsubomi's end that she refuses to have a relationship with Rin unless he agree to marry EVERY girl who's in love with him as well.
In the hentai manga Tayu-Tayu by Yamatogawa, this option is forced on the Official Couple by, basically, all the other girls in the harem.
One of the girls: "Hey, Midori! We're stealing your man!" Midori: (after being screwed into exhaustion) "Aw, hell... Do whatever you want..." (slides to the floor out-of-panel)
Happens in DearS by default, as Takeya ends up the only guy on a spaceship full of horny alien slavegirls in heat, which is going on a tour of the galaxy (two of them — Ren and Miu — were explicitly in love with him while still on Earth and two others — Nyu and China — have expressed attraction).
Often averted/subverted in Sayonara, Zetsubou-Sensei, as every single one of his harem are Yanderes (ironically, except for the First Girl, who's actually the driving force for the harem), in addition to being psychologically "toxic" in some way. Even when they do agree to share him, their consensus concept of sharing is more often than not, literal physical division of the object of their affections.
Until the penultimate chapter that is, when the 12 main girls marry him all at once. It's a bit complicated, however. The crux of the matter is that the First Girl, Fuura Kafuka, had been Dead All Along. Her organs had gone to said twelve girls. What ends up happening is that Nozomu (the titular sensei) sees Kafuka in each of the girls, one at a time (how his sister Rin puts it, he's in love with her eyes, then her lungs, then her vocal cords, etc.) On paper, it's an endless cycle of marriage and divorce between Nozomu and the girls. In Nozomu's eyes, he's always been married to Kafuka and Kafuka alone; in the eyes of outsiders, it certainly looks like this trope, especially since all the girls have children by him... and the kids eerily look like Kafuka). It's also implied that by all the girls donating their blood (metaphorically Kafuka's blood) to her, the journalist that was investigating Nozomu and the girls will join the cycle.
Yomeiro Choice ends this way when the male lead gets over his indecisiveness and realizes that he loves his daughters from the future too much to allow any of them to vanish from the timestream. The series ends with the daughters now arguing about which one should be conceived first.
Asobi ni Iku yo! this is how Eris suggests resolving the love triangle with herself, Aoi, and Manami over Kio. Since Earth doesn't have a law against mistresses it's ok to share Kio. The other girls are ok with it.
In Gintama, Gintoki ended up in one of these situations after a particularly spectacular bender. He ended up trying to set up households with almost the entire female cast including Ototse and Hasegawa, and nearly went nuts in the process. The whole thing was set up by the women in an attempt to get him to stop drinking so much.
In Fujimura-kun Mates, much to Fujimura's dismay he ends up living together with all the girls. Himuro even goes as far as becoming the President of Japan just to change the law to allow polygamy!
Non-romantic example (well, probably): In Soul Eater Not!, Tsugumi, a weapon, has two meister friends vying to wield her in a situation that plays out very much like a love triangle. Eventually, she realizes they're capable of three-person soul resonance and decides to partner with both of them.
In Secret Six, when it was being brought to a (premature) end due to the Continuity Reboot, Gail Simone had its lead, Scandal Savage, marry both her love interests: an exotic dancer named Liana, and a New God named Knockout. Yes, you read that right, it ended with a polygamous lesbian marriage. A prime example of Getting Crap Past the Radar.
In a variation of this, a Silver Age Superman story that saw Superman be split into red and blue versions of himself allowed one of them to marry Lois and the other to marry Lana.
A more disturbing version comes in another Silver Age Superman comic where Lois and Lana find that Superman had been changed into a child and both decide to condition the kid into proposing to them when he's older. It turns out that it was actually a version of Superman from another dimension where polygamy is legal, so he wound up marrying both of them. Shown in this Cracked article.
The concept of the OT3 (or OT4, OT5, etc.) in fanfiction is much like an OTP, but with three or more characters together rather than two. It's quite handy for reconciling opposing ships.
Played with Shinji And Warhammer 40 K in-universe. Shinji's fan clubs are adverse to the idea ("The four points star was not a valid option, dammit."), the Farseer in his head wants this and Rei is practically shoving the idea down his throat.
Played for comedy in DC Nation's Heaven or Las Vegas. The Zamaraons attempt to figure out what makes the Green Lanterns of Earth tick and refine the process behind the Star Sapphire rings. To do this, they take the Earth-based lanterns, and put them in a theme-park version of Las Vegas. While most of the Lanterns wake up in the surreal "test environment" with the woman they're currently dating, Hal wakes up in bed with Carol Ferris, Arisia, "Cowgirl Jill," and Zinda "Lady Blackhawk" Blake — ALL AT ONCE.
Possibly the only option for Rachel Summers in Marvel Nextgen. Current roster of young men courting her include a Shi'ar Imperial Prince, a Logan, and a non-mutant catboy. Franklin Richards is a recurring joke.
In Kyon Big Damn Hero, the odd situation in Hinamizawa was resolved this way. This inspired Haruhi to start aiming for it herself, between all the girls in the SOS Brigade and Kyon.
This is actually deconstructed and then reconstructed in the same chapter. According to Rika, the romantic situation at the end of the Higurashi series turned into a No Romantic Resolution situation for several years until Rika was physically old enough to make her move on Keiichi, but the way she tried to catch Keiichi's notice nearly destroyed her and Keiichi's relationship with Mion and the rest of their friends (and considering that this is Higurashi, it's not hard to imagine how that might've played out). Disaster was only averted at the last moment when a Marry Them All solution was proposed by Rena and Satoko and accepted by the rest of the group. Even then, it's not an easy relationship to have; it only works because the whole group were iron-clad True Companions before they were a harem, and each of the girls has enough self-esteem and love for their partners to not worry about him ever abandoning them.
In Naruto fanon there's something called the The Clan Restoration Act which forces the last member of a important family to marry many women just to increase the clan population. This is mainly done if Kekkei Genkai or special techniques are present in the clan. This is normally applied to Naruto, but occasionally with Sasuke or Uzumaki OC's
On occasion, they mention that females just have to have a ton of kids over a short period of time. Who has the best deal?
The Total Drama Island fanfic Total Drama Luxury Tour is a Egregious example. It's a continuation of the third season, centered on Cody. It had many and many moments of Ship Tease with the other female contestants, and the author even created a poll about which contestant you'd like to see Cody with. Then in the end he gets all five girls anyways: Gwen, Bridgette, Heather, Sierra, and Courtney. And Blaineley also likes him. Even more, the author planned doing the same thing with Noah or Ezekiel as a sequel, although problems with the harem settup in Cody, the author not wanting to break up two of the (three) Beta Couples, and what the remaining girls are like mostly prevent it.
Discussed in Rosario To Kiva - as Wataru is Fangire royalty, he can legally, by the laws of his people marry all the haremettes.
Crossoverpairinglover's fanfiction does this, a lot. Out of all his main stories, only one of them lacks Polygamy (It instead points out one character has Polyandry and gets lampshaded in his comments). It's to a point that the characters aware of the Multiverse actively ask why it happens. Though the writer goes out of his way to explain why it happens and give it logic, as opposed to having the characters blindly accept it like most writers.
Ouran High School Host Club fanfic does this a fair amount, for people who can't decide between the numerous pairing choices offered or just can't see a way for everyone to be happy otherwise. Ranges from Tamaki/Haruhi/Kyoya OT3 all the way up to OT7 fare which includes everyone.
In the Frozen fanfic Sorry About The Mess, this is how the love triangle between Kristoff, Anna and Elsa is resolved, as the Trolls who raised Kristoff believe in Polyamory
It sort of shows up in Familiar Of Zero fan fiction Jus Primae Noctis, but is downplayed. He doesn't appear to have anything with Kirche or Tiffania, but he officially married Henrietta, magically married Louise using the Albionese flowers, and married Tabitha in his home world. He also discusses marrying Siesta, and whether he does or not it doesn't stop them from being together.
The 1926 comedy Hands Up! features Raymond Griffith falling in love with both daughters of a Nevada gold miner. Each sister offers to step aside for the other. Griffith is puzzling about how to resolve the Love Triangle when a stagecoach arrives. A bearded man exits the stagecoach and introduces the sisters to his wife, who exits the stagecoach behind him. Then a second wife emerges, then a third. Five wives come out of the coach before the man is revealed to be Brigham Young. The film ends with Griffith leaving with both sisters in the stagecoach, which has a sign on the back reading "To Salt Lake City".
In the final scene of Horse Feathers the College Widow marries Groucho, Chico and Harpo's characters simultaneously. (Zeppo is left out as his character was Groucho's son, and that would be squicky.)
The film version of Paint Your Wagon has the female lead attempt this during the middle of the movie, living with both of her love interests in a polyandrous marriage. It works for a while, but at the end of the story, she's only living with one of them.
Older Than Feudalism: In the Mahabharata, our hero Arjuna marries Princess Draupadi and takes her home to his mother, who (not knowing about the wedding) tells him to share whatever he's brought with him equally amongst his four brothers. They go through with it, resulting in one of the few gender-inverted examples in ancient history.
In Robert Jordan's Wheel of Time series, Rand al'Thor is bonded as a warder (i.e. more than married...) to all three of his love interests: Min, Elayne and Avienda. This makes for some awkward sexy times, as all three women can telepathically sense one another. Of course, of all the important male characters (and all the important female characters as well except the three love interests), in the end Rand is the only one who doesn't technically get married by the end of the series.
Happens at the end of M.A.R. Barker's The Man of Gold with Hársan, Eyil, and Tlayesha.
In the Honor Harrington novels, Hamish Alexander chooses this when faced with his love for both Honor Harrington and his own wife, Emily. Initially, he and Honor were having an affair with Emily's knowledge and permission (she's very ill and physically frail, to the point where marital sex ain't happening). When Honor became pregnant, they went open. It helped that Honor was also a Grayson citizen, where polygamy is the norm.
Since men are very rare in the world of A Brother's Price, each man not sold to the cribs will marry every sister in a family. This turns out happily for Jerin, since Odelia, Ren, and later and much to his surprise "Cira" are sisters, and he falls in love with their other grown sisters Trini and Lylia after the engagement, too. He also gets along well with and nurtures their younger sisters. This is a world where sisters are raised to be one unit, sort of; for example the country is ruled by the Queens. Jealousy is brought up in the context of the man having "favorites" but doesn't seem to be as big an issue as it would be in our world.
In Courtship Rite: Sure, six is supposed to be the maximum size for a marriage, but the maran-Kaiel aren't exactly sticklers for tradition. By the end of the book, they have enough power and influence to make their own traditions and add both the girl they've always loved and the new girl they've come to love to their five-marriage.
Xanth - Variant: The Magician Humphrey was a serial monogamist, but all of his wives meet up with each other in death and strike up an agreement to come Back from the Dead, one each month, in order to spend time with him. They are okay with this because each woman genuinely loves Humphrey in her own way and he's the type of person who's only tolerable in small doses.
The Kane Chronicles has a variation. Sadie Kane has the hots for two guys - Walt and Anubis. At the end of the series, Walt becomes Anubis' host, and Walt/Anubis enters a relationship with Sadie. This also saves Walt from dying of a curse.
Sword Art Online has a special sidestory entitled There is but One Ultimate Way where Asuna and all of Kirito's female friends basically pull him into the Underworld VR realm to share him, so that they all can tell him how they really felt about him and get married.
At the end of Diane Duane's The Door Into Sunset Herewiss, Freelorn, Segnbora, Sunspark (a fire elemental), Hasai (a dragon), Eftgan, and Eftgan's husband Wyn all enter into a group marriage. Some of them have previously been lovers in either the physical or the metaphysical sense.
Narrator (Terry Jones):(over a black and white picture of the five British Pythons, arm-in-arm-in-arm-in-etc.) Well, things turned out all right in the end. You mustn't ask how, 'cause it's naughty.
In Plain Sight: When a witness is caught out as a bigamist, he seems to want both of his families to merge into one big happy one. The wives and kids do not agree. Pretty much everyone agrees that they don't want to see the dad very much, if at all, but the end of the episode shows the moms letting the kids play together.
In Doctor Who, Amy is dead set on getting both Rory and the Doctor as her "boys". This includes her commanding the Doctor to be her squeeze while she's marrying Rory. Neither the Doctor nor Rory agree. It takes some serious Character Development, though, before she stops flirting with the Doctor and finally starts seeing him as just a friend.
In Animal Crackers, Spaulding proposes marriage to both Mrs. Rittenhouse and Mrs. Whitehead at once:
Spaulding: What do you say girls? Are we all going to get married? Mrs. Whitehead: All of us? Spaulding: All of us. Mrs. Whitehead: But that's bigamy. Spaulding: Yes, and it's big o' me too. It's big of all of us. Let's be big for a change. I'm sick of these conventional marriages.
The stage play Up Against It (originally written as a screenplay with The Beatles in mind as the leads) ends with three of the male characters married to the same woman.
In Mass Effect 1, a renegade Shepard can suggest this as a solution to the Ashley/Liara (if male) or Kaidan/Liara (if female) Love Triangle. The human is not amused, and the game treats it like you chose Liara.
In the final route of Duel Savior Destiny, which is focused on Creanote In the original version it's a straight harem route and doesn't need elaboration, in the epilogue just before Taiga returns from beating up God Crea describes all the other heroines to her son as being Taiga's lovers, much to their surprise. Nobody ever said Avatar exclusively practiced monogamy, did they?
In Jade Empire, a male player character can successfully complete the romance plots with both Dawn Star and Silk Fox.
Parodied in Namco High; while Davesprite keeps teasing the existence of such an ending, it doesn't appear to actually be there.
Davesprite: just means i wont tell you any of the locations of the ten secret easter eggs that when gathered all together unlock the route to the perfect harem ending the characters all have special outfits for that one if you know what i mean ... Davesprite: haha that oughta keep those suckers busy for a while
As mentioned above, while the School Days anime does NOT go this route, if you work for it you can potentially get Makoto to score and stay with both Kotonoha and Sekai.
In Sharin No Kuni if you refuse to enter Sachi or Touka's routes, fail to get enough points with Hinata and finally choose not to reform society with Ririko the girls team up and decide that since none of them could make Kenichi enter a relationship with them, they should just make him follow the path of his father and have multiple lovers. Despite being that sort of game, though, there is no sex scene for this.
Many of the hentai variety will reward the player for getting a happy ending with every girl by making it possible to get an ending where they get all the girls.
The Yoru route for If My Heart Had Wings actually involves Aoi realising that he can't choose between Asa and Yoru... only for the two of them, being extremely close twin sisters who were both prepared to do an I Want My Beloved to Be Happy, to suggest that he just date them both. Aoi is skeptical at first, let alone everyone else, but the situation goes forward with extremely minimal drama or jealousy.
During the later acts of Homestuck, Jane and Jake go insane as a result of Jane engaging Trickster Mode through Caliborn and Calliope's combined Juju. The two of them are very ecstatic about the idea of a polyamorous relationship with Roxy. For her part, Roxy goes so far as to ask Her Imperious Condescension to put her back in jail to get her away from the two, only to fall to it as well.
Girl Genius: The Castle repeatedly suggests this as an alternative to Agatha choosing between Gil and Tarvek. And the castle includes harem quarters. It's implied that this might be the default solution to things for the old Heterodynes: the castle also casually mentions that the Master's bedroom "only sleeps six."
In the Tiny Toon Adventures movie "How I Spent My Summer Vacation," Buster and Babs encounter a family of alligators living on a Louisiana plantation. The rotund father explains that all 3 of his (equally corpulent) daughters wish to marry Buster.
Buster: I can't marry all of them! That's bigamy! Daddy Gator: Naw, son, that's big of ME!
Not a marriage, but in Futurama: The Beast with a Billion Backs Fry falls for Colleen and moves in with her only to discover that she has four other boyfriends who all live together as one big, happy(ish) family. Fry tries to accept the situation but can't take it for long.
And of course, the same movie has Yivo marry the entire universe (or at least, its biological inhabitants).
Parodied in The Simpsons episode "The Front": An Imagine Spot shows Grandpa Simpson as a Western showgirl, preventing a gunfight between two cowboys by proclaiming, "You can both marry me!"