In California and five other states, it is illegal for psychologists and mental-health counselors to engage in “conversion therapy” meant to change the sexual orientation of minors. The Supreme Court on Monday declined to take up a challenge to the California ban, leaving in place a lower-court ruling that upheld the law last year. A few days earlier, U.S. Senator Patty Murray of Washington introduced legislation that would make conversion therapy illegal nationwide. Murray’s bill, which was cosponsored by half of the Senate’s Democrats, would ban anyone from offering such treatment or counseling to anyone, including adults. A companion bill was introduced in the House by California Democrat Ted Lieu and 68 cosponsors.

There is little reason to think that sexual orientation can be changed through therapeutic counseling. Most professional medical and mental health organizations oppose such efforts. The American Psychological Association sharply discourages conversion therapy; it concluded in 2009 that “there is insufficient evidence to support the use of psychological interventions to change sexual orientation.”

The idea that gays and lesbians can be “converted” into heterosexuals — or vice versa — seems hopelessly misguided, if not downright delusional. But when did it become the business of lawmakers, in a nation governed by the First Amendment, to criminalize misguided and delusional ideas?

Murray and Lieu, who call their bill the “Therapeutic Fraud Prevention Act,” maintain that they are targeting not the expression of an idea but the cheating of consumers by means of deceptive advertising and sham services. “It is fraud if you treat someone for a condition that doesn’t exist,” Lieu told The Washington Post. “There’s no medical condition known as being gay. LGBTQ people were born perfect; there is nothing to treat them for. And by calling this what it should be, which is fraud, it would effectively shut down most of the organizations.”

But the “fraud” here is just — speech. What Murray and Lieu want Congress to ban is not a dangerous drug or a risky surgical technique; this isn’t about protecting unwitting victims from being strapped down and lobotomized, or browbeaten into undergoing chemical castration. There is nothing in their bill about coercion or trickery. The law is written to prevent discussion between therapist and patient, no matter how sincere and well-intentioned.

Legislators and activists may regard the very notion of sexual-orientation conversion as pernicious, archaic, and unhealthy. Millions of Americans, gay and straight alike, would deem it offensive and insulting, even hateful.

But the First Amendment would be worth little if it only shielded only non-offensive ideas. Congress and the states cannot censor private discussions about topics they consider odious. Nor can they circumvent the Constitution by labeling speech “therapeutic fraud.”

Conversion therapy may not be good psychology, but that isn’t what underlies the campaign to outlaw it. As Lieu’s words (“LGBTQ people were born perfect”) suggest, the conversion-therapy ban has more to do with gay-rights politics and polemics than with regulating mental-health services. After all, the list of dubious therapeutic techniques is a long one, and virtually none of those techniques has ever been banned by law. Legislators have not been moved to prohibit primal scream therapy as a form of fraud. Nor have they proscribed orgone therapy, chromotherapy, or past-life regression therapy. And if sexual-orientation conversion therapy is fraudulent, then surely astrology and palm-reading — for which plenty of people pay good money — should have been stifled long ago.

It cannot be repeated often enough: You don’t have to respect an idea to respect the right of others to espouse it. In a free society, the temptation to censor an unpopular belief or practice should always be resisted — not just because liberty of conscience is valuable in itself, but because tables turn. It wasn’t so long ago that legislators in some states were trying to pass “don’t say gay” laws — measures forbidding educators from discussing “non-heterosexual” relationships with students or requiring schools to notify the parents of any students identifying themselves as gay or lesbian.

Those who trespass on the First Amendment to ban speech they disapprove of set an example they may come to regret. If therapy to “cure” same-sex attraction is subject to legislative whim, why not therapy to “cure” atheism? If states can order doctors not to talk with their patients about changing sexual orientation, can they order them not to talk about race or politics? Florida’s legislature in 2011 passed a law barring doctors from discussing firearms and gun safety with their patients; violators were threatened with losing their license. Happily, the US Court of Appeals for the 11th Circuit struck down the law last February. But that such a law could be enacted in the first place is a reminder that the lure of censorship is ever-present — and it comes in every shade of opinion.

It is no business of lawmakers whether people seek therapy to help them come out as lesbian or gay — or to help them overcome same-sex attraction. Few personal issues are more fraught and sensitive than sexuality; and an individual suffering from anxiety or distress has every right to seek therapeutic counseling without being second-guessed by Congress or the state legislature.

“Congress shall make no law … abridging the freedom of speech,” commands the Constitution. That applies to speech that isn’t fashionable or politically correct, to speech that powerful interests vehemently condemn, to speech that we find rude and unenlightened. It applies to speech between therapists and patients, too. Even if what they’re talking about makes you want to scream.