But the animal that initiates the sex is also stil lan animal, so it didn't consent to what it is currently doing, either. It's kinda like if two drunks fuck. Both of them are inebriated, so neither can consent.
Going with the logic, both drunks/animals raped each other, so it kinda cancels out.
In zoophilia, only one party can't consent, so nothing is cancelling it out.
Though I suppose, by my own logic, we shouldn't prosecute a drunk man for fucking a horse. That'd be a funny law: beastiality is legal, but only if you have a BAC of 0.08 or above.