Because itsgrowinglikeaweed took this thread as an opportunity to let us know he/she thinks MG is harmful to the planet. Then went on to explain what else he/she does to save our planet's resources and generally leave a smaller carbon footprint. (This was in response to FDD's query whether he/she owned a car in an attempt to point out some hypocrisy). Al Gore would be proud.
Anyway, some folks think (and probably rightly so) that having kids is the most harmful thing you can do to our environment. You are producing more individuals who will use and/or misuse our planets limited resources. So, all things being equal, an individual who doesn't produce offspring is harming the planet much less than an individual who does.
However, this theory presupposes that you aren't having a kid that also happens to be a genius and somehow figures out a way to, say, power a vehicle with human urine and the exhaust smells like fresh baked bread.
I think that was the point being made. I could be wrong.