This is a straw man. There are absolutely people who are concerned about both overpopulation, and individual overconsumption.

I am one. I limit my personal carbon footprint. I do not own a car (and hopefully never will). I rarely eat meat. I live in a cheap, inefficient apartment because I'm living on a student income, but I keep the heat low to try to limit my energy consumption.

I probably won't have kids, because it wouldn't be consistent with my concerns about climate change to bring another high-consuming American into the world.