Two issues about sexuality (or any ethical debate):
1) - Moral status
2) - The legal status
Usually - if someone is illegal, its because that thing is considered immoral (but not necessarily vice versa eg cheating is considered immoral, but not illegal).
So for the homosexuality sexual liberation movement, is really two parts.
1) Homosexuality is not morally wrong
2) Homosexuality should not be illegal.
So some campaigners may have more pressed on the 2) platform - ie "even if what we are doing is wrong - we shouldn't be sent to jail for it'.
And so by the same token, the zoophiles might make the same claim.
But: -Issue of consent- - Regardless of whether homosexuality is considered immoral or not - certainly consent is a given there, but people probably want to claim that the animal does not consent. My reply: See post above. If the animal is not consenting, then it is rape, and therefore wrong. So the cases were zoophilia can be seen as not immoral are the ones where consent is a given. (How about a dolphin the swims up to you with a hardon and rubs it's cock against you?).
More on consent:
The following is more related to case like the peanut butter on the genitals, so the dog licks them.
Pure consent would involve the party involved being 100% aware of the consequences, repercussions, effects of the said action. However no body is 100% aware of anything, so we try to establish a 'reasonable' level of awareness.
In practical (legal) examples:
-If somebody is really wasted - they do not have the ability to consent. (ie. people DO get done for rape, for having sex with really drunk girls).
-If the person is under 16 they do not have the ability to consent to sex; Because a) they are not aware enough of the repercussions, and b) they are more easily coerced.
-Mentally handicapped people do not have the ability to consent - eg. if you convince a handicap to give you a blow job - you'll get done for it.
The issue with being drunk muddies it a lot - how drunk does someone have to be, for them to lose their ability to consent?
Same with handicaps - what level of mental impairment do they lose their ability to give consent to sex?
Now we can start to explore vaguer areas of consent:
What if the woman has (severe) issues with her father/childhood whatever, and is looking for a father figure, and you have sex with her - (ie take advantage of her fragile state) - Is that consent? It becomes totally murky, because everybodys had social experience which affect their judgement in one way or another. In the case of the woman with fatherfigure issues - you could say that it would be immoral to sleep with her, but -shouldn't- be illegal.
With the peanut butter - the dog doesn't perhaps realise the longer social implications of licking it... but at the time is still perfectly happy to. Should this be illegal?