Mating rituals of the North American WASP - Lauren Lipton

After arguing with her boyfriend about his inability to commit, Peggy Adams flies to a friend's party in Las Vegas where, thanks to a night of drinking, she wakes up next to a man she can't remember. They're both fully dressed, but Peggy is horrified. She's never drunk this much in her life and has never even come close to having a one-night stand.

She sneaks out of the hotel room and returns to New York, where she and her business partner are desperately trying to figure out how they can afford to keep their shop once their landlord raises the rent.
Peggy's boyfriend apologises for the fight and gives her a Tiffany box holding a pre-engagement ring. Though it isn't what she was hoping for, she sees the gift as a sign that marriage is in her future. And she's right, because
the next day Peggy gets a phone call from a stranger claiming she's already married - to him! He faxes her the licence as proof: Peggy wed Luke Sunday in Las Vegas. Luke wants an immediate annulment and Peggy is only too happy to agree - until she arrives in Connecticut where Luke cares for his aging Great Aunt Abigail, and the eccentric old woman makes Peggy an offer she can't refuse.