Marxism is also arguably a philosophy about humans should interact, and like libertarianism -- in the sense that you're using it -- it's a philosophy which seems to be rather at odds with what we've actually seen in human society throughout all of recorded history.
The problem is that even if all the stakeholders agree to a given sociopolitical system that agreement lasts for only one generation. The second and future generations consist of people who didn't explicitly sign up for your system, and there are only three options for them. If they have both somewhere to go to and the resources to get there, they can leave, but neither of those are necessarily under your group's control and thus that may not be a viable option. They can stay and submit to The Way You Do Things, which they may not fully agree with no matter how perfect you think that way happens to be. Or they can stay and try to change The Way You Do Things, which is not likely to be successful if The Way You Do Things works well enough for most people.
So the second option is by far the most likely -- and it's not an option those people ever explicitly agreed to. This is the situation most libertarians (and Marxists) face today, but this is not a situation that gets fixed by discarding the concept of the state. Either being born into a society that has rules you didn't explicitly agree to and that only gives you a "take it or leave it," non-negotiable implicit contract for citizenship is coercive, or it isn't.
The problem is that even if all the stakeholders agree to a given sociopolitical system that agreement lasts for only one generation. The second and future generations consist of people who didn't explicitly sign up for your system, and there are only three options for them. If they have both somewhere to go to and the resources to get there, they can leave, but neither of those are necessarily under your group's control and thus that may not be a viable option. They can stay and submit to The Way You Do Things, which they may not fully agree with no matter how perfect you think that way happens to be. Or they can stay and try to change The Way You Do Things, which is not likely to be successful if The Way You Do Things works well enough for most people.
So the second option is by far the most likely -- and it's not an option those people ever explicitly agreed to. This is the situation most libertarians (and Marxists) face today, but this is not a situation that gets fixed by discarding the concept of the state. Either being born into a society that has rules you didn't explicitly agree to and that only gives you a "take it or leave it," non-negotiable implicit contract for citizenship is coercive, or it isn't.