It's kind of hard to say what 'natural' means isn't it? But that vaguely defined new-agey word seems to have reached an ascendancy over more prosaic and scientific words like 'historically.'
Yeah, the word is often used in a supportive way in argument - something is okay and understandable because it is natural. But "natural," used in that sense, can be used to describe anything that occurs in nature.
I wonder if the people who think rape is natural (because it occurs in nature) would be willing to say that male gay sex or bestiality is also natural because it occurs.