There are numerous arguments to disprove that line of thinking. Nobody in their right mind would believe that rape, incest, murder, or cannibalism are morally acceptable or safe. And those all occur in nature. But what about natural being safe? As I said before, health nuts and advocates of organic products claim that natural means safe.
So, what in nature is unsafe? A lot of stuff: Poison Ivy, Poison Oak, and Poison Sumac, for starters. Then we have Tobacco and Eggplant, which contain nicotine. Solanine, the alkaloid toxin behind Deadly Nightshade, which is also found in the leaves and stems of Tomatoes and Peppers. Any plant with the name "Bane" in it is generally going to be poisonous. The Castor Oil Plant, which contains the notorious poison Ricin. Then there's Hemlock, which causes stomach pain, vomiting, and paralysis. Hemlock is best known as the poison used to execute Socrates when he was sentenced to death. Another one is the Strychnine Tree, from which the pesticide Strychnine originates.
And then, there's mushrooms. There are hundreds of poisonous species of mushroom, many of which are deadly. Death Caps earned their name for good reason! And many of the deadly mushrooms look almost exactly like the ones that are safe to eat!
Poisons aren't just limited to plants and fungus, either. Many members of the animal kindom are poisonous too. Snakes are well known for their venom. The skin secretions of frogs are often poisonous or act as a painful irritant. Toads produce a milky cocktail of toxins and irritants when aggravated. Many insects have poisonous bites or stings. All spiders have poisonous bites, though very few species are able to cause more than mild irritation, but the more poisonous species (like the black widow and brown recluse) are very notorious. Scorpions are well known for their poisonous stings, too. Then, there are bacteria. Many species of bacteria make you sick because of the toxins they produce, such as the infamous E-Coli bacterium. And (unless you're a conspiracy theorist), viruses are natural too, and they are almost universally harmful.
If you thought that natural meant safe, are you still convinced?