Interesting one. Regardless of the sustainability question (we, the West, need to reduce consumption; China, India & Africa need to curb population growth and develop in a better fashion) I have no real desire to have kids. I don't find them cute, adorable or want to spend time with them. I like my niece & nephew – they're good fun but too rowdy for me to (want to) cope with all the time. All the "Oh its not the same when they're yours" nonsense – of course its not, I have to deal with it then, sure there'll be plenty happy times but I like my peace, quiet and 'me time'. So which is more selfish – not wanting kids so I don't have to deal with them, or having kids for my edification? Both. Neither.
Add to that I really believe the 'future being a good place to be' is hanging in the balance and it doesn't add up to wanting kids.
Stopping health care after, say, 75yrs old – that would be a better answer. I know its not realistic and way too emotive to deal with, but its the aging of our population which is screwing us over in more way than one right now.