It's a great idea but unfortunately it sounds like it's going to be very hard to enforce.

If you adopt a pet from a rescue group or shelter in CA, that animal has to be spayed or neutered before you can adopt it or if for some reason it can't be then the new owner will receive a spay/neuter voucher to have the animal fixed free of charge at a certain vet. At least this has been my experience when I adopted my cats where I live. We still have an overpopulation problem. One time since the rescue group foster mom knew me, I was able to neuter my RB Starr later on at my own vets at my expense because he had some problems and wasn't able to be neutered earlier.