Can you prove it in all cases? Or do you assume that all cases on incest involve a minor, and minors cannot decide? And if so, aren't you assuming that minors will never gain rights? And isn't this a rather strange assumption, given there are cases of minors gaining the right to "divorce" their parents?
I didn't say I could prove it, but we are discussing opinion and the justification thereof, rather than "proof". However, it is my contention that ALL cases of knowing incest occur where one partner holds some sort of power over the other. This is obvious in cases involving minors, and less obvious, but still very real in cases involving siblings and similarly aged partners. Families are about love, but there are ALWAYS power relationships in them. I don't believe that a brother and sister, say, can equally agree to an incestuous relationship, as one will always be strongly influenced by the dominance of the other. Its just the way families are, as any of us with a sibling can relate to. Even once one attains his majority, the paradigm of these relationships is firmly laid out, age and maturity doesn't change the dominant/submissive elements of a relationship, thus I contend that incest cannot occur with GENUINE consent, since the submissive partner cannot truly be said to be giving full informed consent.
The case of occult incest, which occurs between long seperated siblings or parents, who fall into a sexual relationship before knowing they are related, is a different matter, but a rare occurence, and not really what we are discussing here.
How, precisely, would you argue that a sheep or dog or cat hasn't given "informed legal consent?" There are any instances of people according them rights, so you can't run down the road of "don't be silly, animals don't have rights, nor can they assent to anything."
Animals can have rights, certainly, but this does not make them capable of giving informed adult consent. I simply don't believe that animals have the ability to make an informed choice in such matters, as they simply do not grasp the world in our terms. Humans have the ability to understand the concept of "beastiality", animals do not. Further, in the case of active beastiality (I'm not going to draw a diagram, but as opposed to passive beastiality) I would contend that animals don't even give the appearance of consent, and it is active beastiality that is generally considered when condemning the practice of beastiality.