Contracts never have merely two parties. They are never “private” in the sense implied above. A contract requires a third party to enforce the contract against either party at the other’s appeal. The existence of the enforcing party is a suppressed premise in almost all contracts, and the consent of that third party is rarely explicitly discussed.
Asking for unbounded “freedom of contract” means asking for the existence of a third party who consents to enforce any contract, and has the power to enforce any contract; in other words, a third party that is amoral and omnipotent; one with no objections to any contract terms, and sufficient power to enforce against any party.
The state, in a democratic republic, cannot be such a third party, because it is not amoral — it has moral (or moral-like) objections to some contract terms. For instance, today’s republics do not countenance chattel slavery; even if a person signs a contract to be another’s slave, the state will not consent to enforce that contract.
I suggest that, given what we know about humans, the creation of an actual amoral and omnipotent third party would constitute UFAI ….
The prevailing arguments against it are incoherent for non-vegans anyhow. Nonhuman animals can’t consent? How can it possibly make sense to claim the relevance of consent for (non-painful) sexual activity for a class of animals which can be legally killed more or less on demand for its meat or skin, or if it becomes inconvenient to keep? The consent argument is bogus; the popular moral beliefs against zoophilia are actually not based on a legalistic rights framework, but on a purity/corruption/ickiness framework.