Based on the -Legal- definition of "consent".. by law, nobody under the legal age of consent can give consent. Even those above the age of consent can have their right to consent revoked, sometimes even retroactively, if they're under 18.
So.. Nobody, by default has the -right- to even give consent. Right, let this concept sink in. You are not allowed to give somebody legal permission to have sex with you. Even if you say "Yes please", anybody who you have sex with is legally raping you.
If you are an adult, then you are -given- this right. If you are mentally vulnerable, drunk, high, psychotic, retarded etc, this right can be revoked, and anyone you had sex with during your time of vulnerability technically, legally raped you.
This isn't exactly how things are defined in the books, but I feel like it's an accurate representation of the attitude, or the "spirit of the law".
Now, as far as animals go, they can't consent. Not legally. They haven't been given that right. Animals have no rights, except for animal protection rights. The right to consent is not one of those rights.
That's the way it is. I don't necessarily agree with it, but that's how it is on the books, by law. Right or not, that's what the law is.
Cry all you want about how "animals totally can consent", show me all the videos of animals being horny (really, please do, it's kind of my kink). It doesn't matter. Your arguments are invalid. Legally, they can't consent.
Animals are in a peculiar place within the law. They are living, breathing creatures with thoughts, emotions, and free will. Yet they are treated as objects, resources, property.
This is where a lot of the anti-zoos draw the line. They say "it's wrong because the law says so". In reality, it's the law because people say it's wrong.