Ok, let's put it another way.
I took on a young cat because she was being maltreated by the original owner's child. I was assured that she'd been spayed.
She was irritable, grumpy, hissy and hated everybody with the exception of the other cats (one neutered male and one spayed female), which I assumed was due to her previous treatment.
Couple of months later, her behaviour changed a little and she seemed to have a particular liking for any men. She got a bit vocal, but maybe that was due to being around two other cats that could talk for England. And she became prone to lashing out at the other cats and any female humans. But maybe that was still the trauma? After all, she'd been spayed, so it couldn't be that. Then she quietened down after a week.
Two weeks later, she called, yelled, writhed around, stunk the place out by peeing all over the place, offered herself up to the cats and any random man that came to the door and became very interested in the stench of a neighbour's entire Tom that he was now leaving against our front door every day. So we had to deal with female cat piss smell and tom cat piss smell. The kids started being woken up by cat fights outside the window at 3am. I booked the first available vet appointment for her, which was two weeks' later.
For that entire time, she constantly writhed, howled and was utterly miserable and angry. She didn't sleep, didn't eat, everywhere smelled and we felt under siege from the desperate Tom cats that all gravitated towards our home. We were also miserable as it meant none of us got a moment's sleep - and apparently, once cats reach a certain age, they don't always only have two seasons/two weeks a year of this, they are continually in season all the time.
We took her for her spaying appointment (and got her vaccinated at the same time because, as the vet pointed out, if they hadn't had her spayed, chances were that that their claims she'd been vaccinated were equally untrustworthy). She came back as a chilled, happy, contented and affectionate cat who loved playing with the kids, slept peacefully all night and never peed anywhere she shouldn't. She's curled up next to me right now, 17 years later. If I'd let her out, I probably wouldn't have seen her again, as she was/is a pretty thing and somebody would have taken the opportunity to make some money out of any kittens.
The other cat is disabled - he suffered brain damage in the womb due to his mother contracting cat flu. He can't sit on the floor without falling over. Presumably his mother's owner didn't believe in vaccination either. Or she escaped and caught it from the male that fathered him and the rest of the litter (all but one other kitten died because of this condition, by the way).
You have the power to take that miserable, piss stinking, howling, sleepless existence away from your cat. That existence that potentially condemns her to abscesses, cat flu, FIV, risks inherent in giving birth, having kittens die, possibly dying herself, constantly in danger or feeling that she is in danger. And you'll be able to sleep at night and not be woken up by all the local toms screaming and howling or her yowling to them and having to clean up the pervasive stench of cat piss from both sides of the front door.