I do not require my puppies to be spayed/neutered. However -- I also don't sell my puppies to homes that I feel are not going to be responsible enough to handle and contain an intact dog.
Honestly, I am a person who goes the natural route with most things. Altering, in my opinion is not natural. It doesn't mean I won't do it! My female, Visa, will be 8 next year. She'll be spayed. I don't want her to get pyometra, and I don't like how her hormones affect her behaviour when she in or close to being in heat. Plus it's a pain in the butt to have to watch her every two seconds, to keep panties on her, etc etc.
So I get to see it from both sides. I'm a person that normally doesn't agree with any messing around with hormones... But because of the hassle, it's soemthing I will still consider at a later age (when I feel it's safer).
So I can't argue one way or another. We haven't seen any major studies on the health of altered vs unaltered dogs. We only get to see points from the views of people who are fanatic about one or the other. Obviously there are benefits to both.. Who is right? I will alter my older dogs... And I don't mind if my puppy owners want to do the same.
However, it IS in my contract that the dog must be 18 months or older before being altered. This is because spaying/neutering cuts off the hormones responsible for closing the growth plates -- meaning they are open longer. This can cause a variety of issues with the bones/joints, and I don't want my young wild Belgian puppies getting hurt.
18-24 months is the approximate age they close. My lines mature quickly, so I rounded down to 18 months.
I do offer one incentive -- if you do the minimum health tests (hips/elbows/eyes) and stick the 18 month neutering rule -- you get your health guarantee. If you don't stick to it, no health guarantee. I don't need people bringing me their overweight 8 year old Belgian whose been through several injuries and telling me it has hip dysplasia and they want a replacement puppy. I want OFA to rate the joints when the dog is under 3 years of age. This isn't because I'm some selfish cruel person who wants people to opt out and lose their health guarantee -- it's mostly because I want every single puppy in every litter health certified, so I can be confident that I am breeding healthy dogs, and I can continue with the same lines, etc. It helps me know what mistakes I am making, and what's working for me.
PLUS, the majority of my puppies go to performance homes -- and if the person is so crazy about agility or whatever it may be, I believe the dog needs to be x-rayed anyways. These are very pain tolerant dogs that will handle alot, and break down early if they're pushed beyond their physical limit.