Are some forms of not too intrusive sterilization for a cat/dog, which would allow them to have a sex life? Are these processes too complicated? Are vasectomies or getting the tubes tied viable?
You seem to be under the assumption that cats and dogs feel about sex like humans do. That assumption is totally 100% wrong. In fact, the way humans treat sex (casually, for pleasure and recreation much more often than for reproduction) is extremely rare in animals.
If a dog or wolf lives in a natural pack structure and never manages to be the "Alpha" or pack leader, there's a good chance s/he never has sex in their entire life. In the wild a pack cannot hunt enough food to raise puppies of all the females, so the most dominant female keeps all other females from breeding. The male wants all puppies to be his, and only his natural offspring, so he fights any adolescent male and keeps them from breeding.
Add to that the strange way sex works for dogs and cats...
After ejaculating, the base (or "bulb") of the penis of a male dog or wolf swells so much that it gets stuck inside the female. You may have heard of (or seen) dogs sticking together by their behinds, which is called "Copulatory tie". From an evolutionary point of view that is a positive trait for the male because it ensures that the semen stays inside the female. For both dogs it means rather distress than pleasure and can lead to a big amount of pain and a huge risk because they are vulnerable to attacks while tied. No rolling over and falling asleep for him...
Have you ever felt the rough tongue of a cat licking your fingers? Now think about the fact that they have similar barbs on their penis... Doesn't sound very pleasurable. Wikipedia states:
Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which is a trigger for ovulation.
Sex drive = stress:
Both dogs and cats have a longer hormone cycle than humans. When they are in heat, their hormones are in overdrive, causing cats to stay away from home many nights in search of a partner. Especially male dogs can get anxious, stressed and aggressive during heat. Their hormones drive all of them to a hectic search for a sexual partner and to fight off any competitors. For the species this makes sense because it creates the next generation. For each individual, it means a lot of stress. Even if the animal is kept inside a house or garden and never has the chance to meet a mate, it's still driven by hormones and stressed.
As with all non-trivial health questions about your pet, you should consult a vet. There are different options for both males and females which have different implications for the animal.
All of them lead to the result that:
- The animal cannot produce offspring anymore
- The animal could still have sex (their respective organs for intercourse aren't removed), but will most likely have no interest in it due to lowered hormone levels.
Especially in female dogs spaying at an early age reduces the risk of mammary cancer. For more information please refer to this question.
My sister's dog died of ovarian cancer two months back, and this would have been avoided if the dog had been spayed.
A dog or cat's sex life is 100% stressful and 0% pleasure, so leaving the dog or cat intact leads to stress for the dog/cat and stress for the owner.
The risk of cancer in the reproductive organs of cats and dogs is greatly elevated if they are not spayed or neutered. Vasectomies or tying off the tubes does not help in this case for obvious reasons.