The particularities of Metro’s advertising guidelines have some riders scratching their heads.

Case in point: New ads for the online dating app OkCupid, which appeared in the system last month, featuring clever tag lines riffing on the acronym “DTF” — millennial parlance for casual sex.

Or the ads promoting a Professional Bull Riders event coming to Fairfax next month. They feature, in huge block letters, a profanity related to the digested byproducts of those bucking bulls, with a conveniently placed hashtag and asterisk replacing the “H” and “I.”

The ads clearly comport with Metro’s advertising policies, passed in 2015. Contrary to assumptions, those 14-point ad guidelines do not explicitly ban profanity. But the displeasure of some riders over the ads highlights dissatisfaction with the apparent inconsistencies in what is deemed acceptable for the viewing of Metro customers.

After all, this is the same transit agency that barred the American Civil Liberties Union from posting an advertisement that simply stated the text of the First Amendment. Metro cited a ban on political or “issues-oriented” ads.

It’s also the same agency that has waged a months-long legal battle with the Catholic Archdiocese of Washington to defend Metro’s ban on religious ads and its continued prohibition of an ad promoting holiday charitable giving that features Nativity-themed clip art.

Metro’s aim is to keep anything controversial off trains and buses and station walls, evidenced by its most sweeping advertising restriction: a ban on ads intended to “influence members of the public regarding an issue on which there are varying opinions.”

But at a time when the nation’s collective psyche is more polarized than ever, and on a subway serving the most politically engaged city in America, it seems that everything — even a run-of-the-mill ad — stands a chance at turning into a controversy.

Here, and at other transit agencies around the country, it seems that the common ground on the issues and commercial interests widely considered to be non­polarizing and inoffensive is rapidly shrinking.

Take the commonplace ads for the military contractor and weapons manufacturer Lockheed Martin . . . or for Uber, a company whose very existence and ability to do business continues to spark a hotly contested public policy debate. Are the crux of those businesses not the definition of “an issue on which there are varying opinions”?

And what happens if a person sees a public service announcement about free flu shots sponsored by the city and claims that is an “issues-oriented” affront to the anti-vaxxing community?

“In today’s political landscape, it is virtually impossible to walk by an advertisement that doesn’t offend you, cause you genuine concern, or inspire a debate in your own head about whether it sends the right message,” said Ryan Jacobson, a free speech lawyer at the Chicago-based law firm SmithAmundsen.

Jacobson and his law firm have worked with many transit agencies around the country to draft advertising policies that will discourage litigation — and they also represent transit agencies in court if they are sued by would-be advertisers peeved that their ads have been rejected.

The legal landscape of public transit as a forum for advertising has become increasingly dicey in recent years, Jacobson said, as courts around the country have taken different tacks in determining where to draw the line between free speech and allowances for agencies to craft their own policies.

But more and more, he said, transit providers are hoping to limit ads that would offend large swaths of their riding population and potentially make them want to avoid the train altogether.

“In a world already so polarized, there are few places you can go to avoid the noise associated with political or religious conflict,” Jacobson said.

As it stands, members of the Metro board appear to have little appetite for revisiting the ad guidelines and are largely wary of wading into debate over particular ads that are deemed controversial. Board members contacted for this report all declined to comment, citing the high stakes of the litigation between Metro and the Archdiocese of Washington.

Last month, the U.S. Court of Appeals for the District of Columbia Circuit ruled in favor of Metro, declaring that the transit agency’s ban on “advertisements that promote or oppose any religion, religious practice or belief” is legal and has been enforced fairly.

Ed McFadden, secretary of communications for the archdiocese, said the organization is determining whether it will appeal the court’s ruling. It has another week to decide.

But already, behind the scenes, Metro is preparing for the possibility that the case may wind its way to the U.S. Supreme Court.

Other organizations have also pointed out what they believe is the excessive sweep and vagueness of Metro’s restriction on issue-oriented advertising. The ACLU sued Metro a year ago, arguing that “in its zeal to avoid hosting offensive and hateful speech, the government has eliminated speech that makes us think.”

And, according to Jacobson, that sweep and vagueness in written advertising policies can also make it tougher for a transit agency to be successful in court.

“Transit agencies must tailor their advertising guidelines to ensure consistency,” he said. “Loosely based, catchall phrases within a policy will be heavily scrutinized by judges.”

Even so, Metro Board Chairman Jack Evans has said in the past that the stringent policy is necessary to prevent intentionally incendiary or abhorrent ads from appearing in the system.

The thing is, the existing policy was never intended to be permanent — at least according to Mortimer Downey, former Metro board chairman.

Back in 2015, when the current guidelines were adopted, the Metro board was facing a brewing disaster.

Controversial anti-Islam activist Pamela Geller was trying to place an ad on Metro depicting an offensive caricature of the prophet Muhammad. Board members feared that such a public display of an incendiary image would incite violence — as it had in a Dallas suburb weeks before.

According to Downey, the board needed legal standing to ban the ads, and it needed it quickly. So, in a rush to quash Geller’s cartoons, the board passed advertising guidelines that would ban a wide variety of things. Religious ads. Political ads. And they included a clause banning “issues-oriented ads,” a catchall to provide a cover in case anything else incendiary came up in the short term.

“The decision was basically, ‘If we don’t want Ms. Geller’s ad to be put up next week, this is the way to do it,’ ” Downey said.

He recalls that at the time he recognized that the policy was overly broad, and he didn’t expect it would be the last word. He said he intended to wait until the Geller controversy blew over, and then hoped the Metro board would revisit the policy and come up with more finely tuned allowances and restrictions.

“And then,” Downey said, “I got fired from the board.”

The guidelines that were intended by some to be a stopgap have existed ever since.

For his part, Downey says he would love to see Metro scale back the regulations, and that he would especially want to see allowances for advertising related to political campaigns or issues of political importance.

“By having an across-the-board ban, what’s not getting placed anymore are clear political ads, like those posters at Navy Yard station that used to say ‘Buy more ships,’ or other reasonable political statements,” he said.

“In this town,” he added, “people want to communicate with opinion leaders.”

And then, he said, there’s the financial benefit that could come from allowing political ads once more.

“Frankly, Metro was making a lot of money off of those,” Downey said.

There’s also the money that is spent on constantly defending the advertising policy in court, though others have suggested that lawyers do the work for Metro cheaply because it is considered high-profile.

Still, Downey said, “if they’re getting sued once or twice a year, that costs money, too.”