But as a human being, it's inevitable you'll need to eat. It follows we should weigh the consequences more heavily for inevitable choices. Having children - at least in richer countries - is a choice, and not inevitable.
On a larger scale, it's arguably inevitable that humanity seeks to perpetuate itself. In that sense, having children is not a choice at the species-level. But we have the unique power, individually and collectively, to reflect on whether and how to perpetuate ourselves sustainably.
Finally, the difference between a vegetarian/vegan diet versus say, the Standard American Diet, has orders of magnitude differences in water, fuel, land, and waste products (that 2nd Law thing again). That means the same resource base and ecosystem could support tens to thousands more vegetarians than meat-eaters. A very large vegetarian family would still have less impact than a single meat-eater.
