Every kitten born and homed means that some other cat, already born and living, somewhere, will not have a home. Most of them will end up being euthanised. Not only is kittening dangerous, stressful and painful for the mum cat (and for your wallet if there's any problem - have you got enough set aside for an emergency caesarian if she needs it?) but it is fundamentally bringing more cats into a world where there are already far, far more living than there are safe, warm homes for them.
When it comes down to it, I prioritise the welfare of the souls already living and needing than of those little ones not even born yet. Which is very sad, and very hard, and a lot of people that I like and respect very much have different views.