Del. Robert Marshall and his liberal critics might be appalled by the suggestion that they share anything in common. Marshall ferociously opposes abortion, he co-sponsored Virginia's constitutional amendment banning gay marriage, and he once even tried to prohibit single women from getting pregnant through artificial insemination. But while he and those on the left differ on policy specifics, they share a core assumption.

This year the Prince William delegate wants the General Assembly to take a stand against porn. He has drafted a resolution declaring pornography a public health hazard and advocating a "policy change . . . to address the pornography epidemic."

The resolution is problematic, and not just because it draws no distinctions between, say, airbrushed Playboy centerfolds and stomach-turning torture porn. It makes a variety of declarations that vary from debatable to patently false—e.g., that pornography "normalizes violence," that it leads to "low self-esteem," that it produces "dissatisfaction in marriage" and has a "detrimental effect on the family unit" and that "overcoming pornography's harms is beyond the capability of the afflicted individual to address alone."

Well now. It's true that, thanks to the rise of the Internet, the volume and availability of pornography have grown exponentially. One study in 1998 pegged the dollar value of the "adult content" industry at no more than $1 billion. By 2015, the estimated value had risen to $10 billion in the U.S. and $97 billion worldwide (inflation over that period was 45 percent). If porn were as harmful as Marshall and others contend, then one would expect its attendant harms to have increased as well.

Yet just the opposite has happened. For instance, the Bureau of Justice Statistics reports that the rate of rape and sexual assault has fallen by more than half since 1995. And at the same time porn has spread, divorce in the U.S. has fallen—and is now at close to a four-decade low.

But let's say Marshall is right. For the sake of argument, let's assume erotic material degrades the spirit and erodes the soul and leads to, as his resolution puts it, "emotional and medical illnesses . . . deviant sexual arousal, and . . . difficulty in forming or maintaining intimate relationships."

Even assuming all that, the question still remains: Why is this the government's business?

Conservatives claim to believe in limited government. Some matters, they say, are best left to other institutions such as the church, social-improvement societies like the Boy Scouts, and nonprofit civic groups. To the extent that porn is a problem, it is a problem for those groups to solve—not the government.

Marshall's desire to involve the government with your sexual appetites sounds much like the progressive interest in involving it with your gastronomic appetites. The "food police," as they are sometimes called, have decided that your diet is their concern. They have imposed soda taxes, banned fast-food restaurants in certain locations, and even proposed regulating the aisle placement of products in grocery stores. The FDA is trying to reduce the amount of salt in prepared foods—even though scientists have raised serious doubts about the case against salt. The nation has been subjected to endless hectoring about an "obesity epidemic." First Lady Michelle Obama preaches the virtues of vegetable gardens and their utility in "growing a healthier nation."

Marshall compares pornography to cigarettes: "Before smoking was identified as a problem, at least the recognition that it led to certain pathologies was a starting point to put restrictions on it." Likewise, food scolds are drawing frequent comparisons between the sugar industry and big tobacco.

But government is not supposed to police everything somebody considers harmful—not unless government is meant to be infinite. Government's job is to protect your freedom to make choices for yourself. The inculcation of wisdom is a job for others.

Puritans of the left and right sometimes try to skirt this point by depicting their targets as threats to public health. But a threat to the health of many people is not the same thing as a public health threat. Air pollution is a public health threat, because individuals can be subject to it without their knowledge and against their will. But if a million people decide of their own free volition to eat a 1,420-calorie Monster Thickburger every day—well, it's their funeral. You might think the burger-eater would be happier if he ate kale chips instead. But he thinks otherwise, and you have no right substitute your preferences for his.

Many liberals think otherwise, of course. They mean to make you do what's good for you, whether you like it or not—just like Bob Marshall.

This column originally appeared at the Richmond Times-Dispatch.