Tennis legend Martina Navratilova can't keep off centre court. Having set down her racquet, she is taking up cudgels in favour of gay sheep. Apparently researchers at Oregon State University and Oregon Health and Science University have been tampering with the hormones of sheep to ensure heterosexual behaviour and hence higher reproduction yields for the farmers. In protest, Navratilova, a lesbian activist who moonlights as an animal liberation activist, faxed these institutions the following statement:

"How can it be that, in the year 2006, a major university would host such homophobic and cruel experiments? … I respectfully ask that you pull the plug on this appalling and misguided research. Surely you can find a way to redirect the millions of public tax dollars that are being wasted on these experiments to a more fruitful venture — perhaps by funding a gay and lesbian community centre to foster dialogue and acceptance for people of all sexual preferences?"

This is Martina's beef. As a vegetarian and a lesbian, she fears that people will no longer think that homosexuality is hard-wired if gay rams can be reprogrammed. The "insidious implication" is that homosexuality in humans can be cured. Her contention is that it is "natural".

She has been supported in this by Norway’s National History Museum. This hitherto obscure institution has been featured around the world because of its unique photographic exhibit, called "Against Nature?" The world's first museum exhibition dedicated to gay animals, it "proves" that that homosexual behaviour is not a "crime against nature". Homosexuality, it claims, "has been observed in most vertebrate groups, and also from insects, spiders, crustaceans, octopi and parasitic worms. The phenomenon has been reported from more than 1500 animal species, and is well documented for 500 of them, but the real extent is probably much higher." Despite some protests, school children are getting guided tours.

For anyone who has a traditional view of sexuality, these images, are, admittedly, rather confronting. If animals engage in sexual activity for non-reproductive reasons how can the link between the unitive and procreative ends of human sexual intercourse be maintained as a fact of nature?

However, the answer is easier than it might seem at first blush. Homo sapiens is an animal, but not merely an animal. We have a lot in common with parasitic worms, but there are some differences, too. Our bodily nature is subject to intellectual direction. A human being unites the intellectual and the corporeal, what is rational and what is animal. We get a distorted picture of man when we focus on one aspect to the exclusion of the other. They can never be separated.

Interpersonal relations which are truly human cannot be reduced to physical sensations of pleasure or pain. Anyone who chooses to behave in that way is rightly called an animal. When he acts like a sex-obsessed parasitic worm, he becomes less than human. This is the fallacy underpinning the Norwegian exhibition. Its logic can be expressed as follows:

Homosexual behaviour is observable in animals.
Therefore, homosexuality is in accordance with animal nature.
Man is an animal and therefore homosexuality is in accordance with human nature.

Now try extending this argument to other aspects of human life. Animals don't take care of the elderly — should that lead us to close down nursing homes? Cannibalism can be observed among animals — should we sell unwanted babies as sausage filling? Most people would say that we shouldn't. Humans are different.

And let's take a closer look at the word "unnatural". Are homosexual acts an intrinsic part of an animal’s nature? The Museum’s zoologists are prepared to stick their neck out and say they are, at least for giraffes. The reasons why male giraffes behave this way are not well understood, but they are certainly not the fruit of conscious, rational decisions. The behaviour of animals is largely governed by stimuli and instinct; they lack the human linguistic ability to be able to "talk through" their emotions. But humans can keep their desires in check and even train them into virtues.

Amongst animals, stimuli can lead to unusual behaviour. Tomcats kill their kittens after receiving “mixed signals”. The hunting instinct is so strong and so hard to switch off that dismemberment and even snacking on their own kittens may ensue. Similarly, the strength of the sexual instinct leads to some quite odd behaviour if it does not have a normal outlet. However zoologists may decide to classify such stray behaviour in animals, human reaction to sexual stimuli involves a good deal of channelling and self-control.

Amongst animals, sex can serve a purpose other than reproduction, such as diffusing tension in social situations. Humans accomplish the same thing with handshakes and smiles. If they were to behave like bonobos in their workplace, there would be a torrent of sexual harassment suits. According to some zoologist, dolphins pack rape. Is that "Against Nature?" Not for dolphins maybe, but it sure is for rational beings.

Due to our rational nature comparisons with non-rational animals collapse at the level of interpersonal relationships. Even at the level of biology, human rationality must be taken into consideration. Humans choose to follow hormonally fuelled drives; sheep have no choice about it. Martina Navratilova and her Norwegian fellow travellers are on shaky philosophical ground if they want to use animals as human role models.

Richard Umbers lectures in philosophy in Sydney.