I run an animal charity and one of our main missions is to encourage people to get street animals (as well as their own pets) neutered. We provide financial subsidies to help them. And yes, I call myself a Buddhist (but if "KIndness" were a religion, I'd much prefer that). In the course of running this charity, we have seen unneutered animals suffer from pyometra (infection of the womb) and obstructed labour, both which cause agonising death, both of which can be prevented if the female animal had been spayed. We have also seen testicular and prostate cancers, ovarian cancer, breast cancer, etc., many animals dying on the streets due to "natural" breeding, entire litter of infant street animals dying of diseases such as distemper, tick fever, parvovirus, etc. Dogs and cats, unlike humans, do not menopause - they continue breeding until the day they die. In Malaysia, local councils practise capture-and-kill as their method of control of the street animal population.
So, as a Buddhist (if I may call myself one), my take on neutering is a resounding "YES" if our intention is to prevent further births and more suffering on the street.
In living this mundane life, there is the ideal and the practical course of action. I believe to strike a balance between the two, compassion is the answer.
Here is a recent case that we encountered: http://myanimalcare.org/2013/07/07/an-o ... og-spayed/
(you may want to scroll to the last paragraph).