is it morally wrong to have sex with an animal?
Is it possible for the animal to consent (i.e. by not struggling etc...)?
A child cannot consent because their full understanding of sex is yet to develop, whereas a fully grown pig knows as much about the concept of sex as it ever will, so you cannot say it is harmful to the pig to have sex with it (assuming it is not hurt or traumatised in the process)
Unlawful sex between humans is a lesser crime than murder, whereas killing an animal is fine, but having sex with one is illegal.
I say in principle I think animal sex is morally ok, but I find the concept disgusting and so I'd keep it illegal for reasons of taste.