As Bob Barker would say...

As a child, I never understood the importance of spaying and neutering animals. But I remembered former "The Price is Right" Host Bob Barker giving everyone this advice at the end of his show.
I thought to myself, "If everyone spayed and neutered their animals, they would eventually become extinct and we'd have no more puppies or kittens!"
However, when I got older and realized how irresponsible some people could be with their animals, I understood Barker's advice.
Some people feel that if their female animals get pregnant, they can simply drop off the offspring to the animal shelter. Wrong!
Instead of being reactive, animal owners should be proactive and get their animals "fixed."
Many local animal shelters and humane societies operate solely from volunteer work as non-profit organizations. In addition, many of them are severely overpopulated — the amount of animals in need of loving home alarmingly excels the amount of willing adopters these animals desperately need. According to americanhumane.org, 3.7 million animals are euthanized each year due to overpopulation.
However, this could be prevented through spaying and neutering. (And adopting from animal shelters and humane societies... But that's a different post for a different day.)
Spaying is the procedure that takes place in female animals that prevents them from reproducing.
Neutering is the procedure that takes place in male animals that sterilizes them. Both procedures are performed by removing reproductive organs.
Although it may seem expensive to have this procedure performed on your animal, many local humane societies offer it at a lower cost. As long as there were no complications, my local humane society offered to neuter Charlie for $35. However, this was not the case.

Charlie was very sleepy for the first couple of days after being fixed.

As Charlie was approaching his first birthday and I had spent a pretty penny on the furniture in my house, I decided it was best to get him neutered to prevent him from spraying and an unplanned kitty pregnancy between him and Lily. I tearfully dropped him off at the mobile unit and was instructed to pick him up after they called to tell me he was ready. Later that afternoon, I received a phone call informing me that they could not perform the procedure on Charlie because he suffered from a double cryptorchidism; which means that neither of his testicles had descended into his scrotum. I would have to take him to a veterinarian to complete the procedure. He wouldn't be able to reproduce, but he would still be able to spray and it was likely that he could develop cancer from it. Although Charlie wasn't technically spayed, he was fixed the same way.

Charlie's face stayed swollen for about a week afterword.

Recently, Lily was spayed and everything went normal for her.

Lily was sore and lethargic for about three days after being spayed.

Although I waited until they were older, it's possible to have the procedure completed at a younger age. Weight also plays a factor in whether or not the procedure can be performed. 
Having your animals spayed or neutered prevents health problems later on, such as uterine and breast cancer in females and testicular cancer and an enlarged prostate gland in males — not to mention the elimination of aggressive behaviors and the annoying whiny meow of a female cat in heat.
I can tell you first hand that since they have been fixed, they have been sweeter and gentler. Sure, they are more likely to become obese, but through other preventative behaviors on my part it can be avoided. I don't think it's wrong to take away an animal's right to reproduce.
I think it's wrong to euthanize innocent animals due to overpopulation.