I didn't have any kids.
Its a message that unfortunately most people still refuse to hear (much less accept) - the earth is massively overpopulated. It doesn't matter how think try to dice the onion, there are far too many people for the earth's finite, renewable resources to sustainably support. Certainly our wasteful and overly consumptive habits are accelerating this destruction but no matter how we tailor our habits, the fact is that there are simply billions too many.
Some people ignore this obvious issue because its a difficult problem to solve that carries with it a host of economic and ethical issues. Others ignore it due to religious beliefs, and others because they worship at the altar of technology (which they believe can solve every problem). As long as this remains the case, we cannot even begin to acknowledge the issue and seek solutions.
Some argue that the population levels of "developed" countries have plateaued or begun to shrink slightly but that isn't happening remotely fast enough given the rate of degradation of our ecosystem. Unfortunately, at the end of the day, objective reality doesn't care about our economic or ethical concerns and the sustainable resources we all depend on to survive (the birds, the bees, the wildlife, the oceans, clean air, clean water, contiguous forests, ect) will continue to be degraded and will force a solution to this problem whether through famine, war, disease or otherwise.