One of the most important timeless lessons learned, then forgotten, then learned again, then forgotten again, and so on, is Lysander Spooner's famous maxim that "vices are not crimes". And to forget this crucial distinction, is to ultimately lead to numerous evils, from 1) amoralism and nihilism on the one hand, and 2) the most egregiously tyrannical abuses and overreach of government power (from things like Prohibition, the War on (people who use a few particular) Drugs, the 21 drinking age, tobacco bans, media censorship, restrictions on sexual activities between consenting adults, etc. all the way to slavery, eugenics, and sometimes even full-blown genocide) on the other, typically disguised as either protecting the nation's "moral fiber", and/or "progressive" social engineering for the "greater good". In truth, it is anything but moral or progressive. Thus, individual rights must be inalienable, and the government should never stray from its original mission of protecting life, liberty, and property. Once the natural right to self-ownership becomes usurped by the state, the very foundation for individual rights ceases to exist, and of course that invites all kinds of trouble, as history has so painfully shown time and again.
To quote Spooner, from his Vices Are Not Crimes; A Vindication of Moral Liberty (1875):
"Vices are those acts by which a man harms himself or his property. Crimes are those acts by which one man harms the person or property of another…In vices, the very essence of crime—that is, the design to injure the person or property of another—is wanting."
"Unless this clear distinction between vices and crimes be made and recognized by the laws, there can be…no such thing as individual right, liberty, or property; no such things as the right of one man to the control of his own person and property."
"If these questions…are not to be left free and open for experiment by all, each person is deprived of the highest of all his rights as a human being, to wit: his right to…ascertain for himself, what is, to him, virtue, and what is, to him, vice…If this great right is not to be left free and open to all, then each man’s whole right, as a reasoning human being, to 'liberty and the pursuit of happiness,' is denied him."
"What man, or what body of men, has the right to say, in regard to any particular action, or course of action, 'We have tried this experiment, and determined every question involved in it…not only for ourselves, but for all…And, as to all those who are weaker than we, we will coerce them to act in obedience to our conclusion?'"
"[A] government, formed by voluntary association, would never have been thought of, if the object proposed had been the punishment of all vices…nobody…would voluntarily submit to it. But a government, formed by voluntary association, for the punishment of all crimes is a reasonable matter; because everybody wants protection for himself against all crimes by others."
"It is a natural impossibility that a government should have a right to punish men for their vices; because it is impossible that a government should have any rights, except such as the individuals composing it had previously had, as individuals. They could not delegate to a government any rights which they did not themselves possess."
"Nobody but a fool or an impostor pretends that he, as an individual, has a right to punish other men for their vices. But…everybody has a natural right…to defend his own person and property against aggressors…And government has no rightful existence, except in so far as it embodies, and is limited by, this natural right of individuals...It is only those who claim that government has some rightful power, which no individual or individuals ever did, or could, delegate to it, that claim that government has any rightful power to punish vices."
"To punish men for their vices…is a sheer and utter absurdity for any government claiming to derive its power wholly from the grant of the governed…because it would be granting away their own right to seek their own happiness."
"Everybody wishes to be protected, in his person and property, against the aggressions of other men. But nobody wishes to be protected… against himself…He only wishes to promote his own happiness, and to be his own judge as to what will promote, and does promote, his own happiness."
"The object aimed at in the punishment of crimes is to secure, to each and every man alike, the fullest liberty he possibly can have—consistently with the equal rights of others—to pursue his own happiness, under the guidance of his own judgment, and by the use of his own property…the object aimed at in the punishment of vices is to deprive every man of his natural right and liberty to pursue his own happiness, under the guidance of his own judgment, and by the use of his own property."
"[People]…must be permitted to control themselves and their property… each man’s life is his own."
We ignore such sound advice at our own peril.
And we know that the late Peter McWilliams, author of Ain't Nobody's Business if You Do: The Absurdity of Consensual Crimes in Our Free Society (1996), would certainly agree with Spooner 100%.
And to avoid any weaseling, dilution, or vitiation, via subjectivism, relativism, and/or denial of agency, of Spooner's (and McWilliams') basic maxim, we should clarify that only acts that objectively harm the person or property of nonconsenting others should be considered crimes in a free society.
UPDATE: People will undoubtedly ask, so what about children, or young persons below the legal age of majority? Well, as John Stuart Mill had famously noted, such a maxim would not apply directly to them, at least insofar as consent is concerned. But even then, to impose criminal penalties on the children or young people themselves (as opposed to the adults involved) for acts that are otherwise legal for adults while simultaneously denying them any agency to consent to such acts is the very height of hypocrisy. And regardless, whatever age we set as the age of majority, for anyone above that age the maxim would apply completely, full stop. In other words, in a free society, once you are an adult, you are an adult. That is something most of the world understands.
Also, as Emile Durkheim famously said, "When mores are sufficient, laws are unnecessary. When mores are insufficient, laws are unenforceable". From another angle, that is a good argument against the other side's specious claim that our society is currently somehow too immoral, weak, fractured, spoiled, undisciplined, individualistic, heterogeneous, or whatever to trust people with personal liberty and bodily autonomy.
And finally, there is that classic Pigouvian argument from externalities. The best response to that is that it is literally impossible for humans to exist at all without imposing both positive and negative externalities on others to one degree or another. So arbitrarily designating select vices as crimes is both over- and under-inclusive. And externalities are generally better dealt with via education, persuasion, and mutual consent, or failing that, via targeted taxes and subsidies. Prohibition is far too blunt an instrument to deal with most externalities, and typically does far more harm than good in practice.