Freedom to make any sort of arrangement as long as all parties are willing. A "contract" would be a formal agreement. If you bring force into the mixture you'll end up with more problems than if you don't. You can't have everyone and their grandmother making arbitrary agreements and then using state power to coerce others into following through, so let them make arbitrary agreements and sort it out amongst themselves. Otherwise you get as much injustice as if you'd just allowed the government to dictate your affairs on a whim.