For the USA the China way perhaps makes sense. But most countries are primary site of one or more core nations, and the state is created to promote and nourish that nations. The constitution says that. This obviously includes watching how many children are born and making sure there are enough to pass the culture forth.
By the way, this is right because you having children makes us all richer in the long run; you driving around on a nice imported car or having a slightly larger house do not.
I disagree. Rampant demography is one of the cause for poverty in many parts of the world. While you can say it makes sense to have 1-2 children to pass to the next generation, having 20 children does nothing good does not make anyone richer in the end. Booms in demography always have detrimental effects, because ressources and economies do not always scale well to deal with it.
>> You agree to pay for it by being born.
I fail to understand how you can "agree" on something by being born, since being born is nothing volontary, while an agreement means there is conscience behind it. Your sentence does not mean anything at all.
There are a lot of places where you agree without actually agreeing. See taxes, citisenship and EULAs.
About your second comment: maybe it's just about words, but technically speaking you do not "agree" with your citizenship, you are just born with it. It's like having parents, you do not "agree with it", it's just given. And this is different from EULAs where you do "agree" (since you have to signify your agreement explicitely) while the legality of the EULAs is, in general, highly questionable.
Why can't we just let in more Chinese immigrants if there's not enough people? China already has plenty of people who want to have more children without me having to pay them to do it.