We would rather spend our time helping make earth a better place rather than raising children which will ultimately consume more. Also we didn't feel it would be right to bring a person into this world only to face his or her additional impact towards global warming and the depletion of natural resources, especially a person who we would dearly love. This is to say I wouldn't want to be brought into this world today. Also when you consider the quantity of resources used by a person over a lifetime, there is a huge savings to natural resources by not having a child. Instead we have adopted the old Crow Indian tradition that couples who are childfree have all the world's children to help care for because they are the only ones with the extra time for this work.
Most parents are too busy to work on the larger issues.
Also some couples delay childbirth till they are in their 30's which helps slow down demographic momentum . In a free world, we must not tell people how many children to have. It is their basic right to determine their family size. However, with rights come responsibilities, and with the right to family size comes the responsibility to learn about OP and how it is affecting the economy, the environment, and human health and then to make responsible family size decisions. Hope I have not been a bore.