I see that while I was away celebrating Christmas, Pope Benedict decided, as Time put it, to take "a subtle swipe at those who might undergo sex-change operations or otherwise attempt to alter their God-given gender." Here's what he said:
"What is necessary is a kind of ecology of man, understood in the correct sense. When the Church speaks of the nature of the human being as man and woman and asks that this order of creation be respected, it is not the result of an outdated metaphysic. It is a question here of faith in the Creator and of listening to the language of creation, the devaluation of which leads to the self-destruction of man and therefore to the destruction of the same work of God. That which is often expressed and understood by the term "gender", results finally in the self-emancipation of man from creation and from the Creator. Man wishes to act alone and to dispose ever and exclusively of that alone which concerns him. But in this way he is living contrary to the truth, he is living contrary to the Spirit Creator. The tropical forests are deserving, yes, of our protection, but man merits no less than the creature, in which there is written a message which does not mean a contradiction of our liberty, but its condition. The great Scholastic theologians have characterised matrimony, the life-long bond between man and woman, as a sacrament of creation, instituted by the Creator himself and which Christ -- without modifying the message of creation -- has incorporated into the history of his covenant with mankind. This forms part of the message that the Church must recover the witness in favour of the Spirit Creator present in nature in its entirety and in a particular way in the nature of man, created in the image of God. Beginning from this perspective, it would be beneficial to read again the Encyclical Humanae Vitae: the intention of Pope Paul VI was to defend love against sexuality as a consumer entity, the future as opposed to the exclusive pretext of the present, and the nature of man against its manipulation."
The Pope might have based his remarks on revelation alone, presenting them as one of those things -- like baptism -- that aren't supposed to make sense to unbelievers. In that case, I would have found them distasteful, but I wouldn't have questioned his argument. However, he's presenting his claims as something he learns by "listening to the language of creation". And that's just wrong.
It is not true that the natural world teaches us that marriage is between a man and a woman -- it doesn't have teachings on the subject of either human or divine institutions, and it surely does not teach us that homosexuality is unknown in nature. (The Pope is reputedly very smart and intellectually curious; did he somehow miss the stories about gay penguins, fruit flies, bonobos, and even, topically enough, black swans?) Lots of fish change sex, as did this ex-hen. There are male animals who act like females, and vice versa.
More to the point: so what? Lots of things that we find immoral are widespread in nature. Spiders eat their mates, for instance, but that doesn't imply that it's OK for us. Lots of things we think are just fine are unknown in animals -- number theory, for instance, or blogging. If you want to argue about what we learn when we "listen to the language of creation", you need to explain how we distinguish it from, say, the language of prejudice. Does the fact that the purpose of eating seems to be nourishment imply that it is immoral to drink diet soda? Does the fact that we 'naturally' get around using our legs imply that we were wrong to invent the bicycle, or, for that matter, the wheelchair? Does the fact that we are born vulnerable to a whole host of diseases mean that we should not develop vaccines and cures?
Personally, I think that the idea of defining what's "natural" for human beings is generally confused. What's natural is often contrasted to what's cultural, but human beings are social animals. If anything is natural for human beings, it is being raised by other human beings, and learning things from them: if we tried to find out what's 'natural' for human beings by dropping an infant into an unpopulated wilderness, we'd have to conclude that what comes naturally to us is starvation.
Likewise, human beings are generally curious and ingenious. When we invent things that are not found in nature, are we doing something unnatural, or using our natural capacity for problem-solving? If we decided to abjure every attempt to innovate on the grounds that it was unnatural, would there be anything natural about that decision? I don't think so.
That said, I'm sure there must be some discussion in which there would be a point to making claims about what's natural to humans and what's not; and in which it would be interesting to try to listen to the voice of creation. But, as I said, one would need to be very careful not to confuse it with the voice of bigotry or prejudice.
One sign that someone is not so much as trying to listen to the voice of creation is getting obviously relevant facts about nature wrong, say by asserting that animals do not form homosexual relationships or change sex. Another is making claims about what's natural without any apparent awareness that someone might find his life unnatural -- say, if he had taken a vow of celibacy, and lectured other people about the unnaturalness of their sexual lives without any trace of irony.
And one sign that someone might be motivated by something other than his Christian duty would be if he preached about the unnaturalness and sinfulness of a group of people who have suffered a great deal of persecution without taking care to warn his followers that whatever Christ thought about being transgender, He surely frowned on cruelty and injustice, and that violence against people who are gay, bisexual, or transgender is flatly wrong.