There's a lot of disagreement over that word "voluntary", though. People who emphasize the "capitalism" part, as in anarcho-capitalists, tend to take a very loose view of what constitutes voluntary agreement, which is basically anything short of initiating actual physical violence, or perhaps direct threats like putting a gun to someone's head. Others, from the more left-leaning parts of voluntaryist circles, require a stronger definition of voluntary.
The standards of traditional western contract law for what counts as a voluntary agreement (i.e. not one entered into under coercion or duress) are somewhere in between.
The standards of traditional western contract law for what counts as a voluntary agreement (i.e. not one entered into under coercion or duress) are somewhere in between.