You are not sure you can prevent accidental breeding.
Your dog is a mixed-breed.
Your dog has health problems and/or behavioral issues that could be passed on to offspring.
When spay/neuter is required by law.
False Myths Regarding Spaying or Neutering
You are not a qualified breeder, but want your dog to have "just one litter" before being spayed. This just adds to the overpopulation problem and has not been proven to be medically beneficial. Don't become a backyard breeder.
You have found homes for all of the pups. If you know people who want puppies, send them to local animal shelters and rescue groups. For every puppy you place in a home, another may be getting euthanized.
You think your dog will get fat and lazy. While hormonal changes can have some effect, weight gain and changes in energy level are also part of the normal aging process. Give your dog a healthy diet and plenty of exercise to prevent obesity.
You want your children to witness the miracle of life. Your dog does not exist merely to teach your kids. Contact a responsible breeder or a farmer in your area and plan an educational field trip with the kids.
You worry your dog will miss its "parts." Neutering does not make male dogs feel emasculated, and spaying won't leave your female with some sort of emptiness. Animals simply do not think this way. It's unfair for humans to anthropomorphize animals this way.
Alternatives to Spay/Neuter
Unsterilized dogs need to be confined to prevent accidental mating. Certain companies have created garments that act like chastity belts for pets, but these will not stop the pets from trying to mate. It's important you understand that the urge to mate can make it very difficult to control some dogs, especially males. Dogs may jump, climb, or dig under fences to reach a mate. You must be very careful to avoid an accidental pregnancy if you want to avoid adding to the pet overpopulation crisis.