Basic Trope: A single character fakes having a spouse or significant other, usually to avoid embarrassment.
Straight: Alice, who was formerly the most popular girl at Troperville High School, doesn't want to admit that she's single, and so she takes Bob (her cousin) with her to her reunion, telling everyone he's her husband.
Exaggerated: Alice and Bob wear a fake or borrowed wedding band set to the reunion, and present a forged marriage license.
Downplayed: Alice gives a fake married name at the reception and gives an excuse for why her husband isn't with her.
Alice doesn't want the other women losing respect for her because she's single.
Alice doesn't want the men that she didn't like then (and doesn't like now) hitting on her.
Alice is a lesbian, but still in the closet because of what others might think.
Alice is Bob's real wife, but he treats her as though she were merely his friend or sister, for one reason or another.
Alice is in a relationship, but goes to the reunion alone and tells everyone that she's still single.
Zig Zagged: Alice goes alone to the reunion and tells everyone about her husband who is home with the flu. Everyone thinks she's making him up, but he turns out to be real. (Triple Subverted)
Alice goes to the reunion alone, and does not claim to have a husband or boyfriend.
Alice is truly in a relationship, and goes to the reunion with her real partner.
Enforced: The writer is trying to show Alice's insecurities as a Christmas Cake, a closeted lesbian, someone who's ashamed of her real husband, or whatever else the plot demands.
Lampshaded: "Did you say 'husband' or 'cousin'?"
Invoked: Alice is concerned about her image, so she decides to start traveling around with an attractive male friend or relative and claiming that he's her husband.
Alice knows that lying only causes more trouble, so she's honest about her single status, her sexuality, etc.
Alice confesses the truth, and learns that she didn't even have any real reason to be ashamed; no one loses respect for her because she's single.
Discussed: "How'd Alice get a boyfriend on such short notice? I think something's up."
Conversed: "How exactly do you explain to someone that you need them to fake being your husband?"
Deconstructed: Someone is bound to be hurt or embarrassed (or both) by all this deception.
Reconstructed: Fortunately for Alice, the lie is never found out - because Bob lives far away, and Alice loses touch with her former classmates (either on purpose or by accident) after the reunion.
Played For Laughs: Alice is married, but ashamed of her husband, and takes her attractive and wealthy cousin along to the reunion in an attempt to impress her former classmates. Her real husband shows up, but no one believes he's really her husband.
Played For Drama:
Alice's secret comes out in a most humiliating way, causing everyone to lose more respect for her than they would have lost if she'd just been honest from the start.
The man that Alice chose to pose as her husband happens to be the boyfriend of someone there, which (understandably) pisses that person off.