Neutered cats. This seems counterintuitive. Sex is healthy for people, so it makes sense it should be for cats. And sex is fun and fulfilling, and if you love your animal, you want it to have fun and be fulfilled. This just isn't the case.
There are lots of reasons. Whole male cats mark their territory and they mark it with urine. They back up to the spot they want to mark and raise their tail straight up, and you see the tail kind of vibrate which means they are loosing a stream of urine on a spot they marked before but it seems to be losing its pungency (like your cherrywood antique desk leg, or on that basket of freshly folded laundry you just set down for a minute while you cleared some space in your drawers). Whole male urine has hormones and markers in it that neutered male urine does not. It is pungent like fresh pine, but it doesn't smell at all like fresh pine. You can actually smell a whole male before you see him, from 4 or 5 feet away. Neutered males lack this scentsational characteristic. They also are much less aggressive, and do not need to practice death bites on your hand when you rough house with them a little, because they aren't going to be shredding or getting shredded in fights over ovulating queens.
This dovetails right into the health issues. Whole male cats get ripped up; they can come home half dead (I know of one that came home with his viscera dragging on the ground), with ears shredded or bitten off and even eyes dangling out of the socket. In order to make their contribution to the gene pool, they have to roam, and in the process of roaming, they can get ko'd by coyotes, raccoons, cars and nasty people; they can get into poisons; they can stop for a snack on a small animal that ate poison.
In addition, sex itself poses a real risk for them. There are two diseases among feral and free-roaming cats that some used to say are at epidemic proportions: feline leukemia and feline AIDS (this latter not transmissible to humans). Both are transmitted through bodily fluids: sex and blood (fighting and making love), and feline leukemia is transmitted by licking an infected friend or drinking or eating from the same vessel. Both diseases are deadly, leukemia heartbreakingly so. AIDS takes longer.
There is a vaccination against feline leukemia, and it is something like 60-75% effective, which ain't 100%. And the leukemia vaccine has been directly linked to fibrosarcomas (cancers) at the injection site. The vaccination must be given annually, and so each time the animal gets the injection, it is put at some risk. There is no vaccine against feline AIDS.
Free roaming queens are as at risk for these diseases and dangers as free roaming toms. And their babies will be infected and will die very young, as kittens.
Finally, there is the ethical issue that ASPCA and the Humane Society and all the other august animal rights and rescue organizations bring to bear: There are so many animals that need homes. I don't know the current statistics, but around 2000 here in NYC where I live, 40,000 animals were "euthanized". When your little guy or little lady makes babies, they are filling slots in homes that some of the little sleepers would have gotten, so basically, when you don't spay or neuter, you are putting other animals "to sleep".
All that being said, I will say this on a personal note. I rescued a kitten once, a little nursing male who could not have survived the night had I not taken him in. He in turn and in ways I needn't share with you, saved my life. He is gone, and I would have to say I would give almost anything to have one of his sons or daughters.
You really must spay or neuter your cat. But if you should decide to have just one litter, do it in a controlled environment. Choose the mate, keep them both inside, let them have-to until her estrus stops, get the male neutered. And as soon as the babies are weaned, get the queen spayed. Otherwise you put your best buddy at risk.
Hope this answers your question. Good luck.
Experience and every cat care book and manual I have ever read.