http://en.wikipedia.org/wiki/World_population
It only took a decade to go from 6 to 7 billion. We can build up a population in the blink of an eye.
2) Do parents have children to bring more tax payers into the world? :) Ponzi scheme or not, people taking the time to establish themselves first means fewer children, but also fewer neglected children.
You say it like it's a silly idea, but not long ago people in America had children expressly for the purpose of helping around the farm, and in Asian cultures there is still an explicit overtone that parents have children so that they have someone to depend upon in old age.
And by 'explicit overtone', I mean your parents say it to your face and you become a social pariah if you abandon your family- at least as best I understand it from my Asian friends.
1) Due to social values, we build momentum. If marriage ceases to happen and children cease to be born, 50 years from now we won't have 50 million fewer people- we'll have 1 billion fewer people, and it will take decades to reverse the trend. (made-up numbers, but I hope you take my point)
2) It's not just about protecting the species. We have a tiny bit of a Ponzi scheme going. The young support the old, and as a result graphs like this: http://japanfocus.org/data/fig3.jpg (Japan's population) As a taxpayer, make your blood run cold.