Breeding Brides is an all-time favorite. A young couple want to get into the cuckold lifestyle, and they meet up with a more experienced couple to watch the wife take on a gangbang unprotected, with the aim of getting pregnant. The young wife gets so turned on she ends up joining in. Neither of them fall pregnant, so they try again — this time wearing their wedding dresses. This web site has part of the story; ASSTR has all four parts:
https://www.asstr.org/~Kooba/