Arthur: Francine and Arthur simultaneously fulfill this trope in an early episode.
Done in the first episode of season 3 in Atomic Betty. When Betty finds out her best friend Noah has been kidnapped, Sparky makes a quip about going to save her "boyfriend", which prompts her to respond with this trope. Also comes with the obligatory Ship Tease during the rescue of someone they think is Noah (but is actually a disguised infiltrator).
Sparky: "He's not her boyfriend." Yeah right! [turns to X-5] Would you kick that much booty if that were me dangling up there?
Zuko snaps "She is not my girlfriend!" at a waiter who calls a girl he was on a date with his "girlfriend", though it's not exactly an unreasonable assumption. Oddly enough, when Jun once called an archenemy his "girlfriend" a season earlier, he barely even reacted.
Although they reacted far more strongly to Jun's accusation in the finale.
Jun: I see you worked things out with your girlfriend. Zuko: She's not my girlfriend! Katara:[simultaneously] I'm not his girlfriend!
Also in the second episode:
Sokka:[referring to Aang] We're going to save your boyfriend. Katara: He's not my... Sokka: Whatever.
French version of the above:
Katara: He's not my boyfriend. Sokka: Not yet.
Also happens when Toph meets Jet for the first time (not having joined the group until after his first appearance):
Oddly subverted in the Beetlejuice cartoon. Over the course of the series, the title character and his best friend encounter a number of characters who misinterpret the nature of their relationship, sometimes explicitly referring to her as his girlfriend. Neither one ever bothers to correct the misconception; then again, Word of God has indicated that it might not bea misconception. This could be implied in the debut episode when Lydia is about to kiss Beetlejuice until she sees the bugs in his teeth.
In the episode "Plumber's Helpers", when Helen calls Kevin Gwen's boyfriend, she angrily denies it... despite the fact that a few episodes earlier, she was complaining that Kevin wasn't asking her out.
Happens earlier in the series, too, in "What Are Little Girls Made Of?", though not as drawn-out. Gwen lets Ben and Kevin climb into her room through the window, and her father asks if he's her boyfriend. They both quickly reply "No!" and Gwen blushes.
In another episode, Ben simply asks her where Kevin is, and Gwen launches into a full-blown rant about how he isn't her boyfriend.
Chowder: "Chowder's Girlfriend" was devoted to Chowder trying to convince Panini that he's not her boyfriend! Since that episode, he yells that at her if she so much as says "Hi!" to him. Oddly, most likely due to serialized series and a tendency to be aired "out of order", it's his response to her in the first aired episode as well. Even more strangely, at the end of the series Chowder is married to her... or so Panini would lead the audience to believe.
In Code Lyoko, Yumi and Ulrich frequently state these kinds of denial, mostly spurred by Odd's innuendos or, in Yumi's case, her brother Hiroki's hounding. Yumi also says "He's not my boyfriend" about William, but in this case it is genuine, especially once he becomes an enemy. Yumi manages to invert, then play the trope straight, in just one line:
Yumi: Yeah? Well, it's not the same thing. I'm Ulrich's girlfriend! I mean, I'm—I'm his good friend.
In her debut episode, Nigel tells his friends this about Lizzie, who actually is his girlfriend, because he's embarrassed. After this episode, however, he stops hiding it.
Also Wallabee and Kuki (Numbuh Four and Numbuh Three).
Numbuh Four: We're going to save her, and this time It's Personal! Numbuh Two & Numbuh Five: Wally and Kuki, sitting in a tree! K-I-S-S-I-N-G! Numbuh Four: Shut up! It is not like that! [starts blushing] She owes me a quarter!
Danny Phantom: Sam and Danny frequently insist "We're not lovebirds", even though they end up becoming just that in the Grand Finale. Also counts as ironic Book-Ends. In the beginning of the first episode, they're protesting vehemently that they do not have a relationship. At the end of the last, they finally kiss.
In the DC Super Hero Girls episode "#HateTriangle", Jessica Cruz is surprised that Star Sapphire thinks that she and Hal Jordan are an item.
Jessica: Oh, wait. You think... [points between her and Hal, starts laughing] Oh, no, no nononononono. [between laughter] No, no, no. No, no. [calms down, get serious] No. No. No. Nonononono! We are not together. Not even close to even being together! We couldn't be less together if— [gets shot at]
The Western Animation series of Earthworm Jim is an inversion of this, where the eponymous mutated annelid insists that the Action Girl Princess Whats-Her-Name (a Cute Monster Girl who is basically an alien bug) is his girlfriend, even though nobody else around seems to think they're together. This includes the Princess herself. Jim laughs at this.
An episode of The Fairly OddParents has a group of aliens mistaking Vicky for Timmy's girlfriend. Naturally, Timmy is repulsed by the idea.
Said by Mac in the first episode of Foster's Home for Imaginary Friends featuring Goo. This was said not only due to Mac being eight years old and embarrassed, but also because it was true, and Goo was being a real pest to everyone. Naturally, no-one believes him. Goo, who is living in her own little world at the time, probably never even suspected that anyone (let alone everyone) thought she was Mac's girlfriend.
On Franklin, this happens with the title character and Beaver's cousin Betty in "Franklin and Betty". They really are just friends, and though the ribbing causes Franklin to briefly break off the friendship, the story ends with Franklin (with some prodding from Bear) deciding that it's okay to play with Betty, regardless of what anyone else thinks. As for Beaver, she's thrilled, as she was becoming annoyed by Betty's moping. This also comes up with Samantha in Franklin and the Turtle Lake Treasure, though by the end of the film, they definitely seem to be in Puppy Love.
Oddly averted in Futurama: Leela wakes up from a dream about Fry (after the latter died), to find his jacket draped over her shoulders. She triumphantly yells "He's alive! HE'S ALIVE!", prompting her neighbor to yell through the wall "Shut up! My boyfriend's alive too!" Although Leela has been rejecting Fry's advances (and in obvious denial) for over two seasons by now, at the moment she's too ecstatic to remember that he's not supposed to be her boyfriend.
Harold and Big Patty shared a Commonality Connection and they hang out together. Stinky and Sid saw them and asked Harold if Big Patty is his girlfriend which got Harold saying this trope in verbatim.
When Helga wrote "Arnold loves Helga" on the wall, she quickly erases her name and put Lila on it because her classmates including Lila are coming her way. This cause them to think Arnold has a crush on Lila which got her to ask him about it. Arnold denied that he had feelings for her until he really does which is after they stopped hanging out.
The Kim Possible Files, a scrapbook published between the show's first two seasons, also has Kim denying the idea of her and Ron as a couple.
Also surfaces between Drakken and Shego in the Grand Finale.
Monkey Fist also states this about DNAmy in "Gorilla Fist".
This exchange from the third season's "Overdue" — it's particularly funny, because Ron says it completely out of nowhere.
Shego: Where's Possible? Ron: SHE'S NOT MY GIRLFRIEND! Shego: Whoa, whoa, whoa! Never said she was. [smirks] Ron:[uncomfortable] Oooooh... sss-awkwaaard...
Really like with Danny Phantom, this show seems to have set a record for invoking this in the first episode. Only Ron said it into a megaphone... while trying to get a date.
Ron: Contrary to popular belief, I am not dating Kim Possible.
The Loud House: Lincoln Loud and Ronnie Anne Santiago repeatedly deny being in love with each other, especially since they've seen how sappy and mushy the relationship between their respective older siblings, Lori and Bobby, can be.
Given the amount of Loves My Alter Ego, Two-Person Love Triangle, andShip Tease going on in Miraculous Ladybug, this trope comes up. Marinette, as Ladybug, values Cat Noir as a partner, but is turned off by his constant flirtations, and typically objects when people assume they are a couple. Cat Noir's civilian identity Adrien likes Marinette enough, but when asked, he insists that they are "Just Friends". Essentially, both teens are crushing on each other while at the same keeping the object of their affections in the Friend Zone without realizing it.
Jenny: Okay... listen up, because I'm only gonna say this once: Sheldon is not my boyfriend! I don't even know him! He's just some crazy, loser, stalker geek! He's not my boyfriend, he's not my friend, he's not my anything!
And Sheldon heard the whole thing. Ouch.
It was bad enough that even the villains of the episode felt sorry for the guy.
Hilariously played in a Ho Yay situation by Ren in Ren & Stimpy "Adult Party Cartoon" episode "Ren Seeks Help". Ren decides to go to a psychologist and speak about his problems with controlling anger and thus his psychical and physical abuse of Stimpy. He starts with his childhood, naturally, and when he gets to the point of meeting Stimpy, he says a few words and then promptly stops.
Inverted in "I Love Lisa". Lisa Simpson to Ralph Wiggum. On a date. On live television. Preceded by a Big "NO!".
Bart: You can actually pinpoint the second when his heart rips in half!
In the Treehouse of Horror segment "The Fright to Creep and Scare Arms", Lisa reads the tombstone of Billy the Kid.
Lisa: 'I dream of a world without guns.' Don't we all, William? Bart: Lisa has a dead boyfriend! Lisa: He's not my dead boyfriend! He's a dead boy that happens to be my friend.
Hilariously played with in "He Loves to Fly and He D'ohs", when Bart teases Homer about his appointment with Mr. Burns.
Bart: Hey Dad! Have fun with your boyfriend! Homer: HE'S NOT MY BOYFRIEND!
In the "Bro-Down Showdown" episode of Sonic Boom, Eggman refers to Amy as Sonic's girlfriend. Sonic denies it, but the game show they're both on indicates a wrong answer. Technically, it was because Sonic and Eggman's answers didn't match, but the timing of the "wrong answer" light is still very suspect.
Eggman: If you fight my robot, we'll be disqualified and you'll never win that couch for that whiny little girlfriend of yours. Sonic: She's not my girlfriend! [the game show light indicates Sonic's answer is wrong]
Patrick had to go undercover (because he thinks someone is going to kick him out of town), so he dressed up as "Patricia". SpongeBob SquarePants had to tell a few folks "she" wasn't his GF.
Star vs. the Forces of Evil has Star and Marco often get their close friendship mistaken for this when they actually have crushes on other characters (Star with Oskar and Marco with Jackie Lynn-Thomas), leading to this line being uttered by either character. However, it later turns out they they actually do have feelings for each other but neither will admit it. Things are further complicated when they start dating other people.
Star Wars: The Clone Wars: A mission brings Obi-Wan back to Mandalore and into contact with Duchess Satine, whom he had previously met during a Jedi intervention many years earlier, and they pick up where they left off, complete with bickering. Anakin quickly picks up on the tension (perhaps due to his own experience with romance after long separation) and ribs him about it, leading to this exchange when fighting off an assassination attempt.
Anakin: I'll take care of this, Obi-Wan. You, go find your girlfriend. [runs off] Obi-Wan: Yes, right... No, Anakin, she's not my... [grimaces]
He undermines that when they both confess love a few minutes later, but while they did and do have feelings for each other, now they are both in important positions and Married to the Job, so they part amicably.
Strangely averted in Steven Universe. Steven's best human friend, Connie, is sometimes mistaken for his girlfriend. Steven is implied to have a crush on Connie, but he never denies her being his girlfriend. The overall implication is that he does have feelings for her but isn't currently interested in pursuing a romantic relationship.
On Sym-Bionic Titan, Octus says these exact words to Lance and Ilana after they catch him with Kimmy. Of course, they kiss and start dating shortly afterward. Also, Lance and Ilana, who are masquerading as siblings, are asked this by their Nosy Neighbor Barb. Ilana calmly states he's her brother upon first meeting her. However, they play this trope a bit straight when Siefert refers to Ilana as Lance's "lady".
Lance:[simultaneously] We're just friends! Ilana:[simultaneously] He's my brother!
Robin really ticks Starfire off when he says this in the episode "Stranded". Can't really hold it against him though, as thanks to her established role as The Ditz, Cloudcuckoolander, being raised by aliens, etc., he naturally assumed she didn't even know what a girlfriend was. He is sadly mistaken, as she is quick to inform him. It actually makes the current situation dangerous, as Starfire's powers depend on her emotions, and being confused makes her unable to use them; not good, seeing as they are on a planet with a hostile environment.
That and the episode "Date With Destiny". Robin invokes the trope name, but this time, against Kitten.
"Stranded" also gives us one of the best responses from someone who speaks broken English. Watch it here, starting at 1:20:
Starfire: I am not your friend? Robin:Uh... Starfire: I am not a girl??
As noted on the show's page under this trope, Robin and Starfire had actually been on kinda-"dates" before. Starfire's belief that they were in a relationship wasn't exactly unsupported.
There's one between Heather and Alejandro shockingly. By the later episodes Chris and Duncan tease them about it.
Voltron: Legendary Defender: In "Taking Flight", Hunk gets defensive when Pidge teases him about wanting to rescue Shay, the Balmeran girl who helped him and Coran in "Tears of the Balmera".
Hunk: She's not my girlfriend! She's just a rock that I met and admire very much.
A twist in W.I.T.C.H.: Irma initially can't stand geeky Martin, but an unintentional romantic moment between them leads him to spread the word around that they're dating. Eventually Irma does develop feelings for Martin, but by then has publicly humiliated him and won't actually make a move. When Big Bad Nerissa comes to school disguised as the hot new girl and casts a love spell on all the boys, Irma has to adamantly remind herself that Martin isn't her boyfriend and she has no right to get mad at him for fawning over Nerissa.
Voice: ...but how will he survive, protecting the love of his life, the beautiful Crimson Witch? Scarlet Witch: That's Scarlet Witch, you stupid freak! And I don't need protecting! Nightcrawler: And she's not the love of my life...! [awkward silence]