The word natural is similar to the words normal and truth in that they can only ever be subjective and each person will have their own definition. That what people consider natural or not can be called a fallacy is in itself incorrect simply because of the subjective nature or our relationship to the world around us.
As Morrissey correctly sang "There is no such thing as normal" Correct because normal is defined by each individual for themselves based on many factors. There is no benchmark for normal, just as there is no benchmark for natural or truth, and as such anyone using the word natural uses it from their own perspective applying their own definition.
That said, the word "natural" usually implies what is common in any given setting i.e. it is not natural to see snakes on the streets of London. However, in some Indian towns it is.