Hitchhiking through animal ethics

For reasons that may become clearer, I've been taking an interest lately in Douglas Adams' Hitchhikers' Guide to the Galaxy, and in particular The Restaurant at the End of the Universe.

This book contains what is now a popular thought experiment, relating to an animal that wants to be eaten and can clearly express that desire. You could say it goes against the popular animal rights slogan 'animals can't speak for themselves'. This is a bit dubious, we'll get to that in a minute though. The interesting thing for now is the humans' (loose sense of the word - humanoids?) reaction. Arthur, the hero of the story, is horrified that a talking animal is offering 'its'* own body parts for consumption. It is, he says, 'heartless' to eat an animal that asks to be eaten. At which point Zaphod raises the question of whether it is worse than eating an animal that does not want to be eaten. Thinking about it, this is a very strange response and one which doesn't come up often - Arthur is not a vegetarian that I know of, he didn't object during his 'normal' life on Earth to eating animals and there is no indication that he questioned farming practices. It is almost like the consent acts as a provocation to him to think about these issues for the first time, with a suicidal bovine at the table and the destruction of the universe as a backdrop.

This is strange, as I have tended to encounter the thought experiment in question as something pushed at me by meat-eaters who wish to know if I would a) compromise on my vegetarianism (actually veganism but that isn't the question here) if I were not forcing death on an animal or b) respect an animal's 'right' to be killed and eaten. The answer being, no I wouldn't. I don't have to dirty my hands (well, mouth) to respect a desire I don't agree with.

Of course the waters are muddied when the creature lets slip that s/he has been specially bred to a) want to be eaten and b) be able to express that clearly. Is it really a desire, or even consent, if it has been conditioned? I'd be inclined to see this as a more insidious form of exploitation than standard animal farming practices - and argue, tongue almost in cheek, that maybe it would be better to breed humans with no desire to eat animals!

*I prefer not to call animals 'it'. This is the designation given in the story. I'm repeating it to save guessing about gender.