I have encountered this argument before, but I don't really understand it. In short, it seems to go: We are part of nature, so nothing we do can be counted unnatural. So far as I can see, that is a simple fallacy based upon no more than word play (or, perhaps, a confusion between purely descriptive and broadly prescriptive senses of the term "natural"). The kinds of freaky creatures that now populate the landscape around Chernobyl are part of nature, too, but that does not make them "natural".
Unlike locusts, human beings can choose what they do to the Earth. They ought to choose wisely.
Perhaps you do not share the sentiment, but many people believe that we ought not needlessly drive pandas to extinction not because it is nice for us that there are pandas around but simply because it is a good thing, period, that there are pandas around. This attitude involves a certain kind of resepct for nature that is, it seems to me, difficult to explain and more difficult still to justify. But that, I would suggest, is no criticism of this attitude, which, as both defenders and critics have pointed out, has much in common with relgious faith. Indeed, I'd put the point myself by saying that "creation" is owed a certain amount of respect.
Certainly there are elements of self-interest to this position, too: Perhaps one thinks that a certain degree of humility in the face of nature is worth cultivating; the Earth is now in a certain kind of "balance" without which human habitation would not be possible, at least across very much of the planet's surface. And the Earth has not always been in that particular state of equilibrium. Since the costs of upsetting that balance would, for humans, be catastrophic, perhaps it is best to err very much on the side of caution.