No. This isn't about "controlling the pet population", as Bob Barker would say.
It's about saving your female cat or dog's life. We learned the hard ($$$$) way, in the past 24 hours.
If an unspayed cat or dog is not being bred, chances are fairly high that following a period of estrus (heat), bacteria can find their way into your pet's uterus via the open-during-estrus cervix, and produce an infection. This infection of the uterus is called pyometra, and it's a silent killer of both cats and dogs, because very often the pet exhibits no symptoms, particularly if the cervix remains closed at the end of estrus while the bacteria breed and multiply internally. Eventually the uterus expands as it fills with pus, and can rupture, causing the bacteria to spread even more. The pet can become septic, and even die.
We (well, our cat really was) were fortunate that she did have symptoms. She came to me yesterday morning, meowing loudly. Normally she is not a vocal kitty, except when she wants to play, but this was different, and we weren't sure why. She wouldn't touch her food, although she was quite thirsty. Then she sat on the floor in front of me, meowing some more. When she got up, there was a puddle of red-orange discharge (what is known as the classic "tomato soup sign") on the floor which had spilled from her vagina. She grossed everyone out by promptly licking it up off the floor. It was very alarming, and I knew this wasn't your average "heat", so we called the local vet, and had her seen immediately.
She went for emergency surgery to remove the pyometra. It turns out that her uterus was 4 to 5 times larger than normal, and it ruptured as soon as they removed it. Had we ignored what was happening, or had she hidden herself somewhere and kept cleaning up after herself, she would be dead right now.
We'd always planned on getting her "fixed". It's just that something always seemed to come up, so that the $$ was not readily available. Yesterday's trip to the vet was over six times what a routine spay procedure would have cost. Yes, it was expensive and it cramped our Christmas budget, but we thought it would have been worse for the kids to have had a dead kitty just before Christmas.
Do your pets and yourselves a big favour. Get them "fixed" and keep them healthy.
Love, Scully