To poor people with nothing to look forward to, children are an asset. A few years of hard work, many more years of free labour on the family farm, or (in the west) many years as benefit magnets.
To the rich, children are an immense cost. Loss of one income for several years, about 5 years gross income of the remaining breadwinner per child in direct financial costs, now tuition fees, the prospect they will still be living with you at 35. That's a lot of fun long-haul trips, city breaks, evenings at the theatre (rather than paying even more to sit through Madabloodygascar 9 again), meals out, and lazy Saturday mornings frolicking in bed to give up for the brief pleasure of babaing and gagaing at a new infant. Not to mention the horrible phase, between about teething and 19 years. The weaning period in humans is just too ****ed long.
Wealth is the solution to the population problem but wealth to greenies is the problem numero uno. They totally fail to understand efficiency and that efficiency and being good to the environment go hand-in-hand (well, most of the time). The damage your car does is nothing compared to everyone going around in horses and carts. The sooner the rest of the world is at or close to western standards of living the better.