It's not so much about energy as it is about growing food and consuming soil. While most of the world is still living in abject poverty, we are already over-exploiting some resources (excessive fishing and growing, for example), which is not sustainable. By the time everyone gets to achieve a "western" lifestyle, unless we reduce fertility rates, it's unlikely the Earth will be able to sustain us.

Some of this process seems to be somewhat natural, since fertility has historically fallen in line with economic and social development, but we still need to ensure this remains true. South America, Africa and Asia are slowly "coming online" and if we're not careful we'll be fighting over scarce resources before everyone has had a chance to experience wealth.