We oppose any government attempt to dictate, prohibit, control, or encourage any private lifestyle, living arrangement or contractual relationship.
For any contract to be valid and binding both parties must be at liberty to sign on or refuse to sign on. Pointing a gun at person's head and telling them to sign the contract does not make for a valid and binding contract. In the case of a contract signed under coercion the matter would be settled before the court and justice demands ruling in favor of the person that was coerced.
And under the 13th Amendment prohibition on slavery, contracts for indentured servitude, peonage, and slavery are illegal and unenforceable. But under Libertarianism, once an individual was under such a contract, they could be held captive for life and sold on the auction block like livestock.
Slavery in the name of "freedom" of contract.