Treasure Planet: Doppler reacts this way when BEN thinks he and Amelia are a couple because he's carrying her. It's said with less denial and more tenderness than usually found in other examples, but that's because the film is hinting at their growing fondness for each other.
Ben: Awww, isn't that sweet? I find old-fashioned romance so touching, don't you? How about drinks for the happy couple? Doppler: Ooo, uh—no. Thank you. We don't drink, and, uh—and, uh, we're not a couple.
Subverted in Corpse Bride. When Emily gets taunted about her "boyfriend", she immediately responds that he is not her boyfriend...because he's her husband.
In the Frozen song "Fixer Upper," Kristoff's "family" of trolls immediately assume that Anna is Kristoff's girlfriend when they meet her (after all, she's the first girl he's ever "brought home") and continue blatantly shipping them in song. Though they do falter a bit at hearing Anna is engaged to Hans, they quickly get over it and even try to marry Anna and Kristoff on the spot anyway. They actually manage to get to the vows part before the accidental participants stop them.
Kristoff:Enough! She is engaged to someone else, okay?! (Beat) Troll #1: So she's a bit of a fixer upper. Troll #2: That's a minor thing. Troll #3: Her quote "engagement" is a flex arrangement. Troll Child: And by the way, I don't see no ring!
Scowler: Patchi's got a girlfriend! Patchi: She's not my girlfriend! Alex: Eh, you might be crushing a little. Patchi: Hey, whose side are you on, Alex? Alex: My side.
In Flushed Away, upon Rita's introducing Roddy to her family, one of her sisters teases her about their relationship.
Rita's sister: So, how long have you been Rita's boyfriend? Rita: He's not my boyfriend. Rita's sister: (blows raspberry)
In Capture the Flag: When his grandfather calls Amy Mikes girlfriend, both of them firmly deny this is the case.
Batman: The Killing Joke. On encountering Batgirl again, Paris Franz wants to know where her "overbearing boyfriend" (Batman) is. Batgirl responds with this trope, so it's no surprise that it foreshadows a later Batman/Batgirl Relationship Upgrade.