An example of the Appeal to Nature fallacy is something like this: Animals are not gay, so being gay must be unnatural; therefore, it is bad.
It think it is pretty easy to see that simply because something is “natural,” it is not necessarily “good”. Viruses, cyanide, and cannabis are all natural, yet we brutally slaughter our colds, we seldom serve cyanide Jell-o brand gelatin at dinner parties, and we spend billions per year in an attempt to stop jam-band fans from partaking upon the ganja.
And yet, this “logic” is so compelling to some that they fear recognizing evolution (as the natural process by which humans have come to exist) would require us to enact the strictest types of “Social Darwinism” and to do so would be “right.” This viewpoint is the basis of the failed Ben Stein joint “Expelled: No Intelligence Allowed,” and of the many faith-based decision makers who embrace what is “natural” when it suits them — and vilify it when it does not…
But we are not bound to be mirrors of the natural world; we have both logical and emotional facilities given by nature and which may be used to evaluate nature. We are no more bound to be socially-darwinian than we are bound to be socially-electro-magnetic or socially-quantum. Oh, and by the way, homosexuality occurs in nature.