When Marianna Grace visited Fire Island as a kid, she remembers shuddering at the sight of nudists letting it all hang out at Lighthouse Beach — which at the time looked completely outrageous to the sheltered youngster, who dubbed it “Freak Beach.”

“I always thought, ‘It’s so weird, why would people do that?’ ” says the 35-year-old office manager from Jackson Heights.

Twenty years later, she’s letting her freak flag fly.

“I personally don’t go to regular beaches anymore — it’s close to my heart,” she says, having shed her inhibitions — and her clothes — five years ago after she reluctantly joined a friend there.

The naturist — a fancy term for a self-identifying naked person — warmed to Lighthouse Beach, the pristine stretch of clothing-optional sand 500 feet away from the Fire Island lighthouse. Here, she found friendly folks without judgment.

She thought she had found a new home.

That is, until the National Park Service put the kibosh on nude bathing at Lighthouse Beach last year.

After Hurricane Sandy destroyed a large part of Fire Island’s considerable dunes — essentially the curtain separating the lighthouse from the beach — park officials unceremoniously announced a ban on the clothing-optional section that comprised two 500-foot stretches.

Ostensibly, the ban is about enforcing decency — now that the curtain is lifted, Lighthouse tourists are exposed to the nudists several hundred feet away.

But that’s not stopping Grace from going back again. She’s been at the forefront of the grassroots movement against the ban on naked discrimination called “Save Lighthouse Beach,” which is backed by the Naturist Action Committee, a national organization.

“It’s not like it was obscene, hedonistic culture,” argues Grace. “There were pockets of that, but it wasn’t the norm. My group of friends, we were pretty vigilant.”

She adjusts her sarong, which is slipping south of the border, before adding, “Just because we’re naked doesn’t mean we’re lewd.”

Last Saturday, hundreds of nudists turned out on the shore — some in semi-private camps partitioned off by makeshift curtains, others completely nude or in barely-there bottoms.

Their numbers include a teacher, a police officer, a high-powered lawyer and a Hasidic man named Jon — all of whom declined to be interviewed because of professional reasons.

“My family raised me as a naturist,” says Felicity Jones, a 26-year-old from Long Island City, who showed her support — and her skin — in the wake of the ban.

“There was encouragement for people to go — to show we want our beach back,” she says, in her full splendor. “I’ve never been political before, but this is my whole life philosophy — it’s unjust!”

Why shut down a party that’s been running peacefully for over a century? Since the later 1800s, Lighthouse Beach was a democratic safe haven for nudists of all stripes — including the elderly, straight, gay and, yes, even kids.

But according to a 25-page report released by the National Park Service in February last year, there’s been a recent spike in “lewd, lascivious and voyeuristic behavior, as well as an increase in suspected prostitution, drug use and assaults” at Lighthouse Beach.

Activists deny the claims made in the report, which are based on anonymous law enforcement sources and multiple keyword searches on Craigslist.

“All it is, is hearsay,” says Bob Morton, executive director of the Naturist Action Committee. “It’s inflammatory, but not backed up by any evidence.”

“They never bothered to hold hearings or ask for comments to let [the nudists] know where they’re coming from,” he adds.

When asked for comment, the Park Service declined to elaborate on the report.

Chief ranger Duane Michael acknowledges it’s been a rough road in the post-nude transition, noting that rangers there “routinely still write tickets for public nudity,” which come with a $250 fine, adding that “dealing with these issues throughout the summer are not single-digit [transgressions].”

To get around tickets, most nudists now wear what’s known as a c-patch — a small, paper-thin pouch that covers the genitals but nothing else. (Topless women are protected by state law.)

“Most guys and girls wear a sheer one,” says Grace.

And to show they’re here to stay, nudists are now fanning out from their previously contained sections and spilling onto surrounding family-friendly public beaches.

“Women are going topless in front of the concession stand,” says Eddie C., a 42-year-old naturist entrepreneur from Franklin Square, LI, and a 10-year Lighthouse Beach veteran. “The NPS made it worse!”

For Susan Rothberg, a 54-year-old tax accountant from Lindenhurst, LI, the nudist experience at Lighthouse Beach is far from a perverted pickup scene.

On a recent weekend, Rothberg had schlepped a cart overflowing with Chips Ahoy! and other snacks she had picked up at Waldbaum’s to the beach in a nod to its communal spirit.

“I even brought my mother here a few times,” says Rothberg, who was a runner-up on “Jeopardy!” in 1996.

“The first time I came here [20 years ago], I thought they were going to bring out the scale,” she says, referring to her weight. “But I didn’t feel judged.”

Indeed, say enthusiasts, the great thing about a nude beach is getting right down to the essence of what really matters.

“You shed all the ‘-isms’ when you meet someone naked,” says Grace.

“You don’t know if they’re worth a million dollars or struggling. It’s hard to be superficial when you have no clothes on.”