I will absolutely, 110% agree that if you cannot, or will not, prevent unwanted breeding from occurring, your animal should be spayed/neutered at 18 months of age. [Although really, if you can't prevent unwanted breeding, it's possible you just shouldn't own a dog.]
Most *dogs* surrendered to animal shelters end up there because of behavioral issues [many of which, ironically, can be exacerbated, if not caused, by spay/neuter], not because of unwanted breeding. There have been several, multi-year studies done that disprove the idea that mandatory spay/neuter will reduce dog populations in shelters. While certainly litters of puppies can, and are, surrendered to shelters, the majority of dogs in shelters are there because they have behavioral problems that could have been prevented through training and proper care.
My own experience working in dog rehabilitation for dogs in foster/rescue situations is consistent with the results of those studies.
What would reduce dog populations in shelters is responsible dog ownership. Even though I hate nearly every aspect of the American Kennel Club, I do support and am actively involved in their Canine Good Citizen program, for JUST that reason.
Cat overpopulation, on the other hand, is entirely the result of unwanted breeding, and all cats should be spayed or neutered. I am hoping that the on-going studies of food additives as birth control methods for feral cat populations are as promising as the preliminary results seem to indicate, as it would be a terrific day for those of us who are active in animal rescue.
Regardless, if you as a pet owner choose to alter your dogs, that is your choice, and your right. Note that my initial post was not "My pet peeve is people who spay/neuter their dogs." I don't make a habit of telling people how to live with their dogs, until after they come to me and beg me to help them [which is pretty damned often.]
I choose not to spay/neuter my dogs, and it infuriates me to no end to have people insist that it would be healthier, kinder or in any way "better" for my dog, if I had her spayed. The evidence just doesn't bear it out, nor does my own experience, and if you're going to confront me and insist that what I'm doing is wrong [even though I afford you the respect to leave you to your own devices and choices], you'd better bring evidence to back it up.
Our cats are all spayed/neutered, because we got them while running a Feral Cat Trap/Neuter/Release program.