Appeal to Nature
The Fallacy
Making the argument that because something is 'natural' it is therefore valid, justified, inevitable, good, or ideal.
Why it's wrong
Many 'natural' things are also considered 'good', and this can bias our thinking; but naturalness itself doesn't make something good or bad. For instance murder could be seen as very natural, but that doesn't mean it's good.
Example
The medicine man rolled into town on his bandwagon offering various natural remedies, such as very special plain water. He said that it was only natural that people should be wary of 'artificial' medicines such as antibiotics.