Ultimately, if every action can be justified as moral, which I kind of reject as it just blows up ethics entirely, I'm not sure why they don't argue that throwing yourself off a bridge is, in fact, a net win for the environment. That not only ensures you do not reproduce, here stated as a net win, but you also stop polluting too, and probably feed some sort of aquatic life in the process.

Part of the answer is because not throwing yourself off a bridge is at least comparably moral. Alternatively, having children and teaching them to be good stewards of the earth imbues them with a much higher chance of actually cleaning the place up a bit as compared to said children not existing.