It is much healthier for a female to be spayed before her first heat cycle, it can prevent mammary and uterine cancers and infections. It also reduces the urge to escape outdoors where she could be hurt by cars or other predators.
While we all love our animals like our children, we have to remember that cats are not human. A cat or dog isn't going to feel sad, or feel something is "missing" not having a litter of kittens.
It can be dangerous not spaying a female. If a female continues going into heat cycles, it can cause many problems. Please spay her as soon as possible. There are already many cats/kittens needing homes because of people wanting "just one litter" or their unaltered female gets out of the house and accidently gets pregnant.
If you do want to experience a birth, please go to your nearest rescue or shelter and foster a pregnant mother. You will be helping out, more than you know.
Yes, it would be unkind. If you went to some rescues and shelters right now you would see the hundreds of kittens that do not have homes. It is, frankly, a horrible idea to let your cat have "just one litter."
The desire to have (or not to have) children is a purely human drive; for cats and other animals, it's a biological instinct only, and doesn't have the same emotional component that it does in humans. (Cats also do not have sex for fun, so you're not "ruining her good time." Sex = kittens. Period.) Spayed and neutered cats live full, happy and healthy lives without ever reproducing, and don't feel any loss; it's healthier for them to be fixed, and you'll find it's easier and happier on you, as well. Ask any member here: taking care of a pregnant or nursing cat and her kittens is a lot of hard work, and can be quite costly, in both time and money. Never mind the hard work that goes into finding kittens new homes, when there are already so many cats and kittens out there that need happy, loving homes.
It's not cruel or unkind to spay your cat before she has kittens; it's the responsible thing to do.