"Martin Willett" wrote
Why is sex with animals considered wrong? Is this something to do with the
rights of animals, the moral duties of man or the nature of consent?

I think that it's mainly a cultural taboo, rather than an actual "wrong"
act, assuming the animal is not harmed, like showing the soles of your feet
in China, something like that. The roots of the taboo are likely pretty
obscure, maybe related to early religious beliefs.