OddlyEnough at 21 Jun 2006: 16:54
We have a duty of respect, which is to say that the moraly course of action is to treat others (or perhaps only persons?) in ways that they rationally and informedly agree to and respects their autonomy as persons (this last part is debateable, it more has to do with people selling themselves into slavery than anything else, but I have included it for completeness).
That one does not pleasure an animal does not violate our duty of respect. That its desire is not fufilled does it no harm, so we do not violate a duty of non-maleficence. We might not be fully satisfying a duty of benificence in not sexually satisfying animals, but I believe that in this case, respect outweighs benificence, and so our actual duty does not include such activities.
As a side note, it seems to me that an animal has little justifiable claim to my agnecy in satisfying its sexual desires.
The ball is back in your court sir. =)