Imagine you wanted children and then someone would come along and castrate you because there's a problem with overpopulation. You take away an individaul's choice of reproduction for the greater good. And it makes sense, but the lack of consent or even understandment does not sit well with me.

You're anthropomorphizing animals too much. Cats don't "want" children. Beyond a basic biological need they don't give a shit about procreation. It's not even a difficult question, ethically speaking. In almost every environment they exist in cats are considered invasive, reproduce like crazy, and tend to be incredibly destructive to local species. To be clear, I don't think there is anything wrong with owning a cat, but it's important to do so responsibly.

Your second paragraph is an indictment on how we treat humans not how we treat our pets.

On the fixing side of things, this is traditionally decided throughout animal husbandry by whether or not it positively impacts the animal's ability to do it's job. A housecat's job is to provide companionship and occasional mouse removal for which fixing positively impacts companionship while have no impact on the secondary role. A dairy goat obviously won't be able to produce milk if fixed so they generally remain able to reproduce, but goats kept for their wool are fixed more based on behavior and population control needs. Horses might be fixed to prevent behaviors that affect their ability to pull/be ridden, etc. etc.

So ultimately part of the ethics debate surrounding fixing your pets ties straight into the ethics of animal husbandry to begin with. I personally struggled with this when I moved from the city into farm country and saw first hand how the care for cats flipped from "every cat is special" to "yeah they're just like there and kinda cute plus they kill the mice so I feed them sometimes so they stick around." And I had to grapple with how there is no consistent line for what animal is special and must be saved and what animals are just there and the circle of life is up to them. I don't have any easy answers to give on this subject, so I generally go with whatever is socially acceptable balanced out with concern for the animal's happiness and well-being within the context of them still filling the job(s) they are being kept to fill

In my mind its all about acting in the animals best interest. Childbirth isn't easy on a cats body and neither is the stress they will obtain if they desire reproduction and are not able to do so. Spay and neuter are therefore in the cats (or dogs) best interest