There are several ethical reasons why neutering is hard to justify in a vast majority of cases:

•  It is obviously cruel to put animals through traumatic surgery (since 1985, it has been classed as cruel and unusual punishment to surgically castrate rapists in the U.S.A., even though the eunuchs were given testosterone suppliments). As it is cruel to neuter people who deserve it, it is impossible to say that it isn't cruel to do it to non humans on the grounds that they may want to reproduce.(The word "spay" originally meant to "cut out with a sword", which just about sums up how pleasent the operation must be for the victim).
• Being responsible for an animal being neutered is also a breach of trust between the person and the animal on the grounds that animals rely on us to do what is best for them, rather then going for the most convenient option (animals obviously have no input as to whether they are neutered or not, which makes the whole procedure even more of a betrayal as it is presumably done against the animal's will).
• It is also hypocritical to neuter animals on population grounds when the number of humans on the planet is spiralling out of control (any incidences of humans being sterilised against their will are automatically condemned as human rights abuses, even though it is probably necessary to control the number of people before problems concerning over- population get worse). 
• In Norway, it is seen as being unethical to neuter animals to make them easier to handle: as a result it is illegal to castrate dogs (the term "castrate" is also applied to females in Norway). Admittedly, there isn't a dog overpopulation problem in Norway (which would mean that people would see less of a need to neuter dogs anyway), and it is legal to neuter horses, cats and pigs.    
• Besides, very few humans would want to be neutered against their will, so is it really fair for us to do it to animals?

Another campaign S.A.N. is involved in involves the abolition of the Dangerous Dog Act. The "dangerous" dogs "carers" face a maximum punishment of 6 months in prison and a £5000 fine, while the dog can be executed (or castrated, if the dog is male). Under U.K. law, no human can be punished in this fashion, and it tends to be entirely the "carer's" fault if a dog is aggressive. Therefore, we propose a fixed sentance which involves the carer paying for the rehabilitation of the animal, while being taught how to bring up dogs properly (the cost of this would come directly out of the carer's pocket, of course). We are also in favour of bringing back dog licences so that only responsible people can look after dogs (this should prevent dog atracks occuring in the first place, which would save the Magistrates time).