In jurisdictions where beastiality is legal, then yes, from the libertarian perspective, that's all freedom of contract, baby. I'm not defending either beastiality or libertarianism, but the logic is that you don't want the government deciding what two private entities can and can't freely agree to.

We're pretty far from the Lochner era in the US, where even minimum wage laws were held to be unconstitutional violations of a very broad view of freedom to contract. But it is still a principle in most legal system.