The mas is cuckolded by his father
My 'Spring Break - When it all began' has a scene where a woman has sex with her daughters' boyfriends. Some off-screen swapping between the woman's family and the guy's family is mentioned (but not seen) as well.
It's not the story i am looking for but thanks for the comment anyway, i don't remember if the story happen in a nudist camp or not but at least i want to read something that has a swinging betweeen the mc and his wife with the parents of him