Was perusing an old copy of the Decameron by Boccaccio which is similar to the Canterbury Tales by Chaucer. People often outsmart themselves in those tales and it reminded me of a story with a comparable ending. You'll have to read it to compare.
Joyce, Jim and their 15 year old son Robert go on a two week rafting trip. A sudden rain storm decends upon them and wet and cold they hide in a cave for protection from the weather. That night as Jim makes love to his wife something happens that allows Robert to join in the fun and all three go to sleep, alive, happy and sexually satisfied.
A high-school girl is upset to find out that her male cousin and her aunt is moving in with them. But after they find their parents in a threesome, she gives in to her basic instincts and finds having her cousin around is a lot more fun they she had ever imagined.
A son comes home for the holidays to find his mother recovering from a stroke. He is startled by her child like behavior and lack of inhibitions. He quickly discovers the effects of the stroke have opened new vistas for all of them