Synopsis: There are currently 6 billion of us ravaging our world and whether one is ignorant of ecological reality or not; the fact remains there are limits to how many humans this planet can support.
Those who argue zero population growth is not something we must collectively strive for as a species do so out of their own ignorance. For them, we do not know how many people the planet can support prosperously, so why fret?
Those of us who openly advocate and promote zero population growth do so for ecological and economic reasons. The fact of the matter is there are numerous arguments for limiting the number of humans inhabiting our world.. If one cannot be swayed through ecological arguments for lowering the birth rate, there are economic reasons that are just as persuasive.
First, and foremost, when a nation, species, genepool, what have you, has a large number of offspring for which it must care, these individuals require shelter, food, education, amoung other material needs which place a burden on those carrying them. The smaller the ratio of dependents not in the workforce the less of a load they create. Therefore societies such as Japan or Russia which have relatively small ratios of children under 14 [14-17%] can devote far more resources to the rest of their nations' infrastructure than those with 40% or more such as the Congo or Syria. 
The second economic argument for reducing the growth rate stems from the fact that large numbers of individuals entering the workforce create a downward pressure on wages. This exacerbates unemployment and can again be dealt with if the growth rate is lowered. In poorer nations there are simply far too many individuals seeking too few jobs; having a direct effect on overall poverty.
"Both of these adverse economic effects are reversible by reducing birth rates. With lower birth rates, schools become less crowded, the ratio of dependents to workers declines as does the growth in the number of job seekers. These beneficial demographic effects contributed to the economic "miracles" of several East Asian countries." 
The fact of the matter is quite simple: in order to create a society in which everyone has the ability to reach her or his potential; a world in which at least a handful of the other organisms inhabiting it are not driven to extinction; then One Womb Two Children! must not be just a slogan, it must become a way of life.
These two economic arguments of course, are only a sliver of the complete reasoning for creating agencies which promote contraception, and educate the ignorant in both the industrialized and developing worlds of the necessity of achieving a state of zero population growth worldwide.
The drive in overpopulated nations should be in educating all women of the reasoning for only having two children at this point in spacetime. If they are unable to have children then they should consider adoption as opposed to fertility drugs.
This is very much a "female thing". The "mother" is being "raped" and "violated" by our ravenous species and in order for us to create some sense of balance, the females within our own species need to understand the necessity of why the vast majority of them should have less children.
ASIDE: Within our own species females tend to be more selective of mates. This is for a variety of reasons. One, males can technically have as many children as they can have partners, while females are limited to say twenty children or less in a lifetime. Two, males have very little to lose from sexual encounters and much to gain besides climax. The loss of a little genetic material and energy is far outweighed by the prospect of progeny. Females on the other hand have nine months of pregnancy, the actual birth, and only then does the fun begin. So we can see why females are often far more willing to control promiscuity than their counterparts and why they should be targeted for achieving zero population growth.
We must insure anyone and everyone has contraceptives available when they are needed. There are hundreds of millions [if not billions of people] throughout the world who are unable to purchase contraceptives simply because they are too expensive or unavailable. 
Currently the hierarchies of a number of religions stand in the way of promoting contraceptives in any form whatsoever. Those of us with scientific backgrounds must use scriptural arguments in helping to sway those of a more spiritual persuasion. For example, when advocating zero population growth often we are countered with comments such as, "But God said be fruitful and multiply."
Yes, God did say that, but please tell us how many people did he give this order too? ... We'll give you a hint, ... ... a guy named Noah was in charge. You see simply because God says something at one time does not always mean it is applicable each and every day. God told Jonah to go to Nineveh and speak out against its sin; does that mean each and every one of us should go to Nineveh? Do people go around saying, "God said go to Nineveh!" No of course not, yet you are forgetting when God told our ancestors to be fruitful and multiply he was speaking to eight individuals or less.
For scriptural backing on not having any children please see Luke 23:27-31:
A large crowd of people followed him; amoung them were some women who were weeping and wailing for him. Jesus turned to them and said, "WOMEN OF JERUSALEM! DO NOT CRY FOR ME, BUT FOR YOURSELVES AND YOUR CHILDREN. FOR THE DAYS ARE COMING WHEN PEOPLE WILL SAY, 'HOW LUCKY ARE THE WOMEN WHO NEVER HAD CHILDREN, WHO NEVER BORE BABIES, WHO NEVER NURSED THEM! THAT WILL BE THE TIME WHEN PEOPLE SAY TO THE MOUNTAINS, 'FALL ON US!' AND TO THE HILLS 'HIDE US!' FOR IF SUCH THINGS AS THESE ARE DONE WHEN THE WOOD IS GREEN, WHAT WILL HAPPEN WHEN IT IS DRY?" 
There are those of us in the Alliance yearning to convert to Catholicism, but cannot until the Church openly encourages zero population growth [and opens their library]. In The Hammer of God, AC Clarke fantasizes what a future Pope might say when the Church begins advocating the obvious:
~ "four centuries ago, in the year 1632, my predecessor, Pope Urban the Eighth, made an appalling blunder. He allowed his friend Galileo to be condemned for teaching what we now know to be a fundamental truth, that the Earth goes around the sun. Now, alas the time has come to admit an even more tragic error. Through its stubborn opposition to family planning by artificial means, the Church has wrecked billions of lives --and, ironically, been largely responsible for promoting the sin of abortion among those too poor to support the children they were forced to bring into the world. This policy has brought our species to the verge of ruin. Gross overpopulation has stripped the Planet Earth of its resources and polluted the entire global environment. The goal is to establish the often-discussed but never achieved, except in times of war and plague, zero population growth as quickly -- and humanely --- as possible. Even that may not be sufficient; we may need negative population growth. For the next few generations the one-child family may have to be the norm. The Church is wise enough not to fight against the inevitable, especially in this radically changed situation. Abortion remains a crime, and will always be so. But now there is no longer any excuse --or any need-- for it. AC CLARKE The Hammer of God ENCYCLICAL
 My God, the CIA is good for something! Their propaganda is entertaining too. https://www.cia.gov/library/publications/the-world-factbook/ Interesting site for facts and figures of every nation on the planet. I dod not know how long this link will be good. The morons have changed the link ~five times since I first wrote this. Might as well check an almanac before this one. I will not change to the latest link again. Old site http://www.odci.gov/cia/publications/factbook/index.html
Scientific American, Janus, 2002, Misleading Math about the Earth, Population: Ignoring it's Impact, John Bongaarts. http://www.sciam.com/article.cfm?articleID=000F3D47-C6D2-1CEB-93F6809EC5880000&pageNumber=9&catID=2
[3The Unmet Need for Family Planning. Malcolm Potts, Scientific American, Janus, 2000.
 For those of us more scientifically inclined we must realize that the great religions all have the same foundations for obvious reasons. With the advent of nuclear fission in 1945, our species' own self-destruction became not only a possibility, but in all likelihood a probability. Before the arrival of megatonnage weaponry, the idea of compassion and love for others seemed almost laughable. It still does to the less gifted amoung us. Yet today, ... ... as we gaze across our globe and see the destruction left in our species wake, we understand why extremely advanced civilizations amidst the heavens [God] might whisper to us once in a great while; reminding us that the day will come when we have the ability to destroy ourselves and we had better have outgrown our childish tendencies when it does.