Ok, I know the jokes are coming. 
But, it’s a serious question I’ve wondered about.
Is there a reason why we cut our dog & cat’s testicles off? AFAIK vets don’t offer the option of a vasectomy. Why don’t they?
Would a dog vasectomy still prevent aggression and excessive territory marking like castration? Could Fido still have a sex life with the neighbor’s poodle after a vasectomy?
No, because severing the vas does not impact hormone production. The complete removal of both testes does that, and it is the hormone-driven behaviours that (agression, etc.) which we are trying to prevent.
Would they if you asked them to?
Because humans pay the bills. 
For the converse, you can read the Wikipedia page on Castrati.
It would probably work. However, it’s more expensive, a little riskier, and castration has the added benefit of damping down the aggressive behaviors associated with unfixed male dogs and cats.
Well, eliminating the behaviors is part of what we want. But some of us want to try to reduce unwanted dogs or cats, too.
I’ve heard that in zoos, some lions and other male animals ARE given vasectomies rather than castrations, to preserve the behavior (and sometimes the appearance) of intact males, without causing overbreeding.
Unspayed females have their own behavior problems, by the way (says the owner of two spayed female cats).
My chihuahua got castrated at 7 months old. He still marks territory once in awhile. Gets a boner occasionally too.
I’m sure it would be much worse if he hadn’t been fixed.
Because they don’t make neuticals for humans.
Sure they do.. (Is that NSFW? They look like skinned grapes.)
I know-it was a joke.
Males (pets, that is) are castrated to reduce the annoying behaviour of males - aggressiveness, possessiveness, territory and marking, etc.
Females (pets) are the ones spayed to prevent unwanted pregnancies. I suppose for all the difference it makes, spaying is more definite and less prone to error than tube-tying, and what the heck, if it reduces annoying behaviour like going in heat, so much the better.
Humans seem to have an attachment to their parts and their functions, except during PMS.
One of my classmates in high school had a nice new car, with tiny muddy pawprints up and down it. He said “If I catch that F—ng cat on my car again I’ll cut his balls off!” I had to tell him we tried that, it doesn’t stop them.
If you just want your dog sterilized, but want his sexual behavior intact you can get a vet to do a vasectomy instead of a castration. It’ll just cost you alot more, and you’ll need to shop around for a vet who’s bothing willing & able to do it. You might end up having to go to an actual veterinary school.
Human males only tend to be castrated if they have testicular cancer in both testes, as part of gender reassignment surgery, or in some jurisdictions as punishment for a crime. I’m pretty sure the last one doesn’t occur in the US, except for chemical castration of sex offenders like rapists or child molesters.
Testicular cancer is common in intact male dogs, and neutering prevents the risk. Fortunately, unlike in humans, testicular cancer rarely spreads to other organs in dogs, so if a dog gets testicular cancer, no further treatment is needed after castration. A vasectomy has no preventative measure at all and, like appleciders says, is more expensive and riskier. If you aren’t going to breed your dog, there is no good reason not to do it.
I’ve asked this exact same question before, and the answer I got is that neutering/spaying is not only about sterilizing the animal but that the lack of the hormone producing organs causes desirable behaviour, which is why doing it younger is better before the animal starts displaying the hormonally induced behaviour.
Humans of course are not interested in getting rid of their sex hormones(well most of them) only their reproductive capacity, humans also have body image psychological issues with missing genital organs that animals don’t.
So removing the testes/ovaries in domestic animals is not a “side effect” or just easier, it serves a purpose.
If you paid them? Sure, why not? There are reasons to vasectomize male animals. Stallions that are castrated, for instance, can be used to identify reproductively receptive mares, who are then artificially inseminated.
It also cuts down on the risk of breast cancer (especially if you spay before she first goes into heat) and makes uterine infections and uterine or ovarian cancer impossible.
Give this pup a treat for not writing “dampening”!
This is so – often for animals kept in social groups. Solitary male animals will probably be castrated just like a cat or dog.
Also, vasectomies are a simpler surgery - less time under anesthesia, and quicker healing, which is always important, but perhaps moreso for exotic animals, as vets usually don’t have as much experience with such surgeries.
A vasectomy would only be simpler if the doctor has been trained on it. A veterinarian would likely have less experience with vasectomies than with castration unless he had been practicing exclusively with vasectomies. And if you’re talking about time under anesthesia, horses are domestic animals with well known problems with many types of anesthesia (putting an animal under general anesthesia and recovery usually requires a padded room in many large animal clinics). Yet there are anesthetic IV protocols developed so that surgeries, such as castration, can be done “in the field”. I never did them, but some of my classmates did (during their training in vet school). Granted, there is not as much known of the effects of anesthesia (and such anesthesia as given to horses) in various exotic species.
Also of note, in many places, for females, the option of ovaroectomy (as opposed to ovarohisterectomy) is done. This means removing the ovaries but leaving the rest of the uterus.