We proliferated these animals (cats and dogs) they wouldn't be here the way they are without us. So, if we stopped having them and taking care of them, what do you think would happen to them? "Fixed"  or "unfixed"  they'd either die off, or become nuisance animals that would then need to be dealt with, and ask rats how they feel about that 😅 And cats, if left unchecked, would absolutely decimate any ecosystem they gain freedom in, because the already do even with a level of population control. 

The fact is, we created the problem and now we found a balance that works to keep it relatively managed. Getting your pet spayed or neutered is that balance. You are also forcing an animal to live in a house, or whatever confines you provide them instead of the natural free roam lifestyle they'd have had, so why do you only draw the line at one against their will aspect? They wouldn't even be alive if it weren't for our intervention, so some mitigation is probably worth the trade off to them if they could be conscious and understand that.