I don't know. What possible phenomena can occur that can be called unnatural? 'Natural' is as meaningless as 'organic.'
I agree, but it depends on how the term "natural" is used. If "natural" means only that something occurs in nature, then anything that occurs in nature is, I guess, natural.
Look at the arguments suggesting that rape can be viewed as natural. Isn't that what is being suggested - that rape is natural because it occurs in nature? Are there any other arguments being made in support of rape? I don't think so. I think that natural is just being used here in a descriptive sense that would include anything that happens in nature.
And, in that sense, it is also natural for other people to kill, castrate and/or imprison folks who attempt to use rape as a strategy - because that is what we do with such folks. ;-)
I say heterosexual activity is "natural" because it can produce a "natural" product...another human being. The same is not so of homosexual activity.
On the other hand "orgasms" are natural, no matter how they are arrived at. (pun intended)