By "Voluntarily", I mean nobody is putting a gun to your head demanding that you do or don't do something. Nobody is outright taking your shit or attacking you. And nobody is lying to get you to do something.
The problem most people tend to have with this viewpoint is that they believe there are circumstances where it is appropriate to do this to people, for any reason other than stopping someone who is already doing it or is planning on doing it (self-defense). Usually, this is meant to justify a government's use of involuntary force, although sometimes it might be other involuntary organizations.
So let's go through a list of things a given person may or may not justify being done by coercing others:
Tax collection for public services (roads, schools, police, military, etc).
Punishing victimless crimes (degenerate sex, drugs, opinions that are deemed wrong, etc).
The lesser of two evils: they believe that it's okay for X to coerce against the masses because if they didn't, the boogeyman Y will take over, and they will be much worse than X.
Usually it comes down to those three.
The first two ultimately rely on a core fallacy that these things are inherently 'good' or 'bad', and that because of these qualities, a government or some other institution has a right, if not a duty, to use force against people to suppress or create it. The problem is that words like 'good' and 'bad' are inherently subjective. You can judge things yourself and determine them to be 'good' or 'bad', but you cannot do that on behalf of others. So, an idea that truly is 'good' shouldn't require people to be forced into it, and likewise, something that is truly bad shouldn't have to be made illegal by a coercive institution. The people who agree that something is good or bad can agree, voluntarily, to pay taxes to a certain government and follow certain laws, but nobody has any right to force other people to do the same. Your subjective ideas of good and bad are yours alone, and when you impose them on other people, you are crossing a boundary and violating the free will of another person.
Of course, that forces me to justify why it is 'bad' to violate the free will of another person. Without appealing to any higher authority in the universe, it's simply a matter of convincing you that it is wrong to do so, and we've come full circle. But consider what life would be like if everyone openly justified violations of free will. We would live in a society where the weak, those who cannot fight for themselves, are constantly at the mercy of those who are stronger than themselves. That, in my mind at least, would be hell. It would be something not unlike the lives of peasants in mediaeval Europe, or the outer party and proles in George Orwell's '1984'. Western society has progressed away from (and avoided) that kind of world because we slowly began to see violations of free will as evil, and reject it even when it's done by kings who are ordained by God. It's how constitutional democracies were built, and slavery was abolished. The next step is to reject it once more, even when it's done by a goverment that is voted for democratically.
And therein lies the argument against the third point. History has proven that we don't need a lesser-evil force to protect us from a greater-evil boogeyman, because we, the people, have the power to be the even stronger non-evil force that can do away with both, and it's only a matter of us coming to this realization. It is up to us to decide that no human being should have to suffer the pain of a dog-eat-dog world, by becoming the greater dog and eating nobody.
Go ahead and imagine the society of your worst nightmare, fictional or otherwise. Now imagine how that society could have been established, if it were physically impossible for homo sapiens to aggress against each other. Could it still be possible?
So if you want to become invulnerable to that sort of Tyranny, it's up to you as a member of society to start openly rejecting the violation of free will, and encourage alternative ways to get people to do something. Offer an incentive, appeal to change their ideas of 'good' and 'bad', or just don't force them to do something at all. If everyone did this, everyone would be free, and tyranny or oppression would be impossible.
I'm inclined to agree, however "everything" and "strictly" are I think too absolute, as I do think that there are a few reasonable exceptions when it comes to family. What if, for example, a child doesn't want to go to the dentist out of fear? I wouldn't condemn anyone who coerces them to do so.
I don't think that parents or institutions should have free reign over children as oddly enough I've seen quite a handful of other libertarians suggest, circumcision before age of consent should be outlawed in my opinion unless it's a strict medical necessity and a child in school having to ask permission before they can go to the toilet seems tyrannical to me and goes against the idea of human dignity. But parents do need some leeway when it comes to raising children.