This is a rather long video, by Dr Karen Becker, a holistic vet, as she discusses how her views on spaying and neutering has evolved through years of practice:
https://www.facebook.com/doctor.karen.becker/videos/10154015597777748/
It is about 25 minutes long, but very worth your time to listen to the whole thing so that you understand where she is coming from. But please listen to the whole video in its entirety.
In short, this video is for very responsible pet owners and it isn’t so applicable to those who do massive rescue work.
Pertinent points:
Spaying and neutering at an early age may lead to endocrinal or other health problems in dogs. However, for street animals who do not have the luxury of being protected as an intact dog in a loving home, spaying and neutering take priority over possible health issues (resulting from being spayed and neutered).
Cats, on the other hand, do not seem to have these problems, based on research available so far.
Hence, for street dogs, spaying and neutering are necessary. But if it is a pet dog, you have a choice of letting your dog remain intact, provided your dog is protected in your home and you do not mind dealing with the issues that arise from an intact male or female dog. Dr Becker suggests ways on how to deal with these issues in her video.
So I would reiterate that this video is exclusively for dog owners who can afford the luxury of not neutering their dogs.
As for the street animals out there, neutering is still very, very necessary.