Why should something be a crime?
What are the conditions for something to become illegal? Empirically, if something is enforced by the executive branch of government, it is effectively a law. But should that be enough, from a Constitutional design perspective? What should it take for a preference to become a law?
In democracies, something becomes illegal when a certain percentage of a population is willing to vote for someone to make it illegal. In the U.S., typically, that requires a majority of people in a majority of districts to think something is bad enough to warrant criminal charges, which works out to about 20% of the population (p. 6) (25%, if all districts were the same size). In the abstract, it could vary between a tiny minority not liking something under a dictatorship to requiring a supermajority in a strict constitutional republic. Personally, I think a stong supermajority should be required for something to be made a crime, but getting there can be quite difficult. If 51% of the population is willing to fight over something, the 49% are SOL. Extolling people to respect a constitution can only get you so far.
If the negative externality to an action is greater than the enforcement costs of prohibiting it, the action should be outlawed or taxed. Such logic has an internal logic to it, but ignores the dynamics of political power. Also, the size of externalities are often impossible to measure. If 51% of the population doesn’t like smokers, they can get smoking outlawed, regardless of how much smokers enjoy it. Additionally, it is impossible to figure out exactly how much people are subjectively hurt or helped by such a law. You can try to do some rough calculations, but there is no real exact method.
Suppose Catholics hate Protestants and are willing to pay $1,000 to outlaw Protestantism, and Protestants only value their freedom at $950, does that justify outlawing Protestantism? Where are the default property rights? In the absence of transactions costs, people could Coasean bargain to the efficient solution, but the initial endowment still has a big wealth effect, and thus a big impact on outcomes. Should a crime require a violation of someone’s rights? Are some rights God-given and inviolable, or are “natural rights” merely a rhetorical technique to convincing people not to restrict your actions? I don’t really know the answer to all these questions. Personally, I like to live and let live, but that’s just not a popular option for most people.