"Unnatural, mummy? You tell me, what's nature's way? If poison mushrooms grow and babies come with crooked backs, if goiters thrive and dogs go mad and wives kill husbands, what's unnatural?"
Richard, The Lion in Winter
One of the claims to which people seem peculiarly susceptible at the moment is that if something is "natural", it must be good. "Natural" foods are believed to be uniformly healthy. The finding that some particular behavior (say, polyamory) is found in nature is taken to be some sign that it is a good thing.
I think a fair amount of this results from our culture having lost a sense of tragic vision in regards to nature -- we naturally assume that unless some active force comes along and makes things bad, that they will be good. This could not be farther from a traditional view of nature. While neo-pagans are sure that being "in tune" with nature would be a blissful and pleasant state, real pagans of the ancient world saw the natural forces that were bound up with their gods as capricious, sometimes cruel, and almost always unconcerned with the impact of their actions upon mortals.
We as Christians see nature as having been created by God and being something that He saw as good. Yet in a fallen world, I don't think we'd be far off in taking a fairly tragic vision, similar to the ancients, of how we relate to nature and what "nature's way" is.
This also comes up in the current debate over same sex marriage, where I've on a number of occasions had people tell me that if attraction towards members of one's own sex is "not a choice" but instead something "natural", then obviously same sex marriage must be a good thing and what God intended. It would be cruel, it is argued, if God allowed some people to have such an inclination but did not allow it to be fulfilled through marriage.
I don't know if I'm just particularly heartless, but I find this mode of argumentation entirely unpersuasive. It seems to me that there are lots of strong, sometimes seemingly irresistible, desires that we have which it would not be moral to fulfill.
A tragic vision seems an essential means of coping with the world as we find it. More Greeks and Norse, please.