Tilli Buchanan says she was just trying to stay cool when she and her husband stripped to their underwear inside their house.

Then her three stepchildren walked in. They seemed embarrassed, so Buchanan took the moment to make a point, she told the Salt Lake Tribune. Their dad was bare-chested. Shouldn’t she be able to walk around topless, too?

That effort to make a point grew into a constitutional challenge to Utah law, as Buchanan, 27, was charged with lewdness involving a child — allegations that could land her in jail and require her to register as a sex offender.

Now a judge has weighed in against her on the challenge, saying that Utah’s law singling out exposed female breasts but not male ones is justifiably rooted in “contemporary community standards of nudity.”

The decision earlier this week from Utah’s Third Judicial District Court adds to a patchwork of rulings on laws against female toplessness that the Supreme Court declined to take up just this month, saying it wouldn’t hear the appeal of three New Hampshire women who were fined for exposing their nipples in public.

Buchanan’s lawyers had cited a ruling by the U.S. Court of Appeals for the 10th Circuit — which has jurisdiction over Utah — that in February overturned a ban on toplessness in Colorado because it treated the two sexes differently.

Women in Fort Collins “risk criminal sanctions for making a choice — to appear topless in public — that men may make scot-free,” the appeals judges wrote in that case, according to the Denver Post.

But Judge Kara Pettit said Buchanan’s case involves different issues than the Colorado effort started by a local “Free the Nipple” group. Buchanan is accused of exposing her breasts in the presence of children younger than 14 “under circumstances the person should know will likely cause affront or alarm” or with an intent toward sexual arousal or gratification. Prosecutors will have to show that Buchanan falls under that category — not just that she had her bra off, the judge said.

As for the argument of discrimination against women, Pettit said it’s up to lawmakers to gauge “community standards” regarding lewdness.

“It is not for the court to decide whether the Legislature’s enumeration of lewd conduct is wise or sound policy,” she wrote.

Prosecutors for West Valley City praised the decision, while a spokesman for the American Civil Liberties Union of Utah — which argued for Buchanan — told the Salt Lake Tribune that the group is “disappointed” and still “consulting with our client about next steps.”

Whether Buchanan will appeal is unclear, the Tribune reported, and if she doesn’t, the criminal case against her will proceed. Her misdemeanor charge is punishable by up to 364 days in prison and a maximum fine of $2,500. If convicted, Buchanan would have to register as a sex offender for 10 years.

That latter punishment could mean that Buchanan would be cut off from her stepchildren, her lawyer Randall Richards told the New York Times. “Anywhere there are kids, she’s not allowed to go,” he said. “It’s a ridiculous charge.”

Richards did not immediately respond to The Post on Thursday morning.

Prosecutors recount the event differently than Buchanan and her lawyers, according to the Tribune: They allege that Buchanan drunkenly commented in front of her stepchildren, ages 9 through 13, that she should be able to take off her shirt if her husband could do so, then removed her clothes. The prosecutors also allege that Buchanan told her husband that she would put her shirt back on only if he showed her his penis.

Court documents say that authorities learned about Buchanan’s removal of her bra during an investigation of child sex abuse, which Buchanan’s lawyer told the Times is not related to his client. The mother of Buchanan’s stepchildren reported the toplessness incident to authorities because she was alarmed, according to the Tribune.

Buchanan has said she was shocked to face criminal penalties when she just wanted to be comfortable, after she and her husband were dusty and hot from putting up drywall in their garage.

“It was in the privacy of my own home. My husband was right next to me in the same exact manner that I was, and he’s not being prosecuted,” Buchanan told the Associated Press last year.

Leah Farrell, a lawyer at the American Civil Liberties Union of Utah, also framed it as a matter of equality in an interview last year with The Post.

“It’s problematic to bring in all the consequences of criminality simply based on our feelings about how a woman should be and how a man should be,” she said.

Correction: The article previously identified Karen Pettit as a federal judge.

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