She Is Not My Girlfriend/Playing With

Basic Trope: Two people who are not in a romantic relationship are assumed to be by others, leading to denials of such by one or both parties.
 * Straight: Alice and Bob are Just Friends. Carol sees them together and asks Bob if Alice is his girlfriend. Bob immediately and loudly denies it.
 * Exaggerated: Alice and Bob simultaneously deny it, and proceed to describe every single reason they would never, ever date each other... seriously! (Of course, they are long overdue for a Relationship Upgrade and Everyone Can See It.)
 * Justified: There are good reasons Alice and Bob would never date; for example, they're siblings.
 * And whenever one looks at them, their Shipping Goggles fall onto their eyes.
 * Inverted: Alice and Bob are already dating, and are asked if they're brother and sister.
 * Alternately, Bob is a Clingy Jealous Boy, shouting to anyone who will listen that yes, Alice IS his girlfriend, thank you very much!
 * Subverted: Alice and Bob are asked if they're dating... and Bob says yes. Cue Relationship Upgrade.
 * Double Subverted: ...except they were just pretending to be dating and later on go back to denials.
 * Bob says yes, much to Alice's shock.
 * Parodied: Bob has to deny being romantically involved with Alice, Carol, Dana, Ellie, his sister Francine, his mother Georgia, and Harry.
 * Alice and Bob were making out when Carol walked up to them and asked if they're an item. Bob is insulted at the accusation!
 * Deconstructed: Thanks to constantly having to deny having romantic inclinations towards Alice, Bob starts to resent the time spent with her and starts distancing himself from her.
 * Reconstructed: Bob learns that, aside from the jokers just trying to give him a hard time, others are only saying all this because they truly think he and Alice would make a good couple and want him to be happy, or were trying to point him to the fact that Alice does indeed have feelings for him, or noticed Bob's feelings for her and were trying to help him realize those feelings and act on them.
 * Zig Zagged: Alice and Bob seem to confirm or deny their relationship at random.
 * Averted: Alice and Bob are already in a relationship, hence making the point of any denials completely moot.
 * Enforced: "How to establish Alice and Bob as the Official Couple... let's have everyone else assume they're already dating!"
 * Lampshaded: "She's not your girlfriend? Yeah, I've heard that line before..."
 * Invoked: Carol has noticed the UST between Alice and Bob and asks if they're dating to provoke a reaction.
 * Defied: Alice and Bob know of their constantly being linked together, and decide to fake a relationship in order to throw off inquiries.
 * Discussed: "Do you think Alice and Bob are going out?" "Maybe, but I doubt they'd admit it."
 * Conversed: "There they go, denying it again. Can't they just spit it out already?"
 * Played For Drama: Bob denies that Alice is his girlfriend, but Alice, who secretly (or not-so-secretly) likes him and has thought that he likes her back, becomes heart-broken by his statement and thinks he doesn't like her at all. Bob then has to sort out the mess (and his feelings for Alice).

Why don't you go back to your date with She Is Not My Girlfriend now?