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.