Look at it this way; if you're going to argue an animal can't give consent to have sex, you have to acknowledge it can't consent to being killed for food; to being hunted, sometimes by means that give a great deal of pain; to undergoing medical procedures; or to being used as a means of transportation either. And killing it is surely worse than having sex with it.
I'm not justifying bestiality; I'm drawing attention to the peculiar lengths people will go to, to avoid condemning bestiality on moral grounds, while still maintaining its illegality. The 'consent' argument is one such.
Me, I think bestiality is a crude and disgusting act, and should be illegal because beyond a certain point crude and disgusting behavior should be illegal, even if it harms no one.