The Spa Castle in Queens is billed as a sprawling, 22-pool Disneyland of soaks and steams.

But some patrons are anything but G-rated.

On Sunday mornings, an army of exotic dancers getting off their Saturday night shifts descends on the complex — and it’s not to do laps in the pool.

“We open at 6 a.m., and lots of strippers come here after work to unwind,” an employee told The Post. “There’s lots of skinny-dipping and the girls are making out with each other and partying. It looks like the set of a porn movie.”

During two recent visits, The Post found patrons in their birthday suits, groping each other, making out and having what appeared to be underwater sex.

Last Sunday, the clientele ranged in age from toddlers to octogenarians. But that did not stop a group of five heavily tattooed twenty-somethings from engaging in splashy sexcapades in the 2-foot-deep, heated wading pool on the second floor.

House music pulsing, one of the women stripped naked and cheerfully played with her breasts while a woman in a yellow-fringe string bikini and a man looked on. A few minutes later, the nude woman stood in front of the seated man while her hand jerked in and out of the water. A couple made out a few feet away.

A male member of the group wondered out loud if they should make a liquor-store run.

It was not yet 9 a.m.

“There is a big problem with customers having sex in the pools, and the management doesn’t know how to handle it,” said the staffer, speaking anonymously. “Practically every day, couples are caught having sex in the indoor and outdoor pools. They think they are being sly about it, but they’re not. When you see somebody on someone’s lap bouncing up and down and the jets aren’t turned on, you know what’s happening.”

The mega-spa is the brainchild of developer Steve Chon, 56, who came to New York from South Korea in 1980 when he was 21. A self-described “gambler” with “big guts,” Chon passed on working at the family laundry and studied engineering at the Polytechnic Institute of Brooklyn, according to a profile on Inc.com.

For two decades, Chon’s forte was building and renovating Korean-run laundries and nail salons. But in 2005, the developer with champagne tastes purchased a ramshackle warehouse on 11th Avenue in College Point for $3.6 million. He envisioned it as a family-friendly hotel-spa. The spa’s Facebook page has even described its mission to create a “light-hearted, family-oriented” ambience.

The community board promptly shot Chon down, “deriding the proposal as a thinly disguised ‘hot sheet’ establishment,” the Queens Gazette reported.

But Chon gained the support of then-Queens Borough President Helen Marshall, who helped shepherd the project through a series of city approvals.

Today it is New York City’s largest spa, with 300,000 annual visitors and sales of “more than $20 million in recent years,” Chon told Inc.com. He opened a second Spa Castle in Texas in 2012 and a third on East 57th Street in Manhattan in 2014.

The $25 million, 100,000-square-foot complex offers 21 spa pools, a wading pool, hydrotherapy baths, high-tech steam rooms, 10 gold-tiled saunas and services like facials and body wraps.

‘In the outdoor pool I would see lots of girls sitting on guys’ laps and they would be bobbing up and down. It was pretty obvious they were having sex.’ - Whitney Ward, customer

After paying a $40 entry fee ($50 on weekends), clients don knee-length shorts and T-shirts — blue for men and hot pink for women — along with a watch that serves as both a locker key and charge card for food, drinks, massages and facials. For an extra $290, a client can book a private VIP room for a 90-minute massage that includes herb tea, cookies, fresh fruits and a 30-minute aromatic Jacuzzi.

The wading pool is where much of the action occurs. It’s heated and equipped with underwater jets and a swim-up bar. Drinks like the Spa-tini (gin, Chambord and pineapple juice) or the Brainstorm (rum, Disaronno and pineapple juice) run $18 a pop.

There is also a kiddie pool, and the steamy behavior often occurs even as children roam the grounds.

“I had to stop going on Saturdays because it was like a full-on, urban high-school orgy in the outdoor pool upstairs,” said Whitney Ward, a 42-year-old Manhattan photographer. “Under-the-water mushroom . . . in the outdoor pool I would see lots of girls sitting on guys’ laps and they would be bobbing up and down. It was pretty obvious they were having sex. I’ve also seen guys rubbing one out the wet way.”

One spa-goer saw a young girl smuggled into the men’s dressing room. “When she changed clothes, I was shocked to see breasts,” wrote a Yelp reviewer in November who identified himself as John l from Queens. “I did a double take. ‘What the hell?’ I covered up and told the staff, but they did nothing.”

David Chan, 46, a Manhattan retail manager, said he saw two guys “rubbing each other off” in a steam sauna in the locker room. “Now I have this picture in my mind of other men’s sperm swimming around in the hot tubs,” Chan said. “This is supposed to be a place where you can bring your family.”

A.S. Bunny, 63, a retired nurse from the East Village, said he was once propositioned. “This tiny Korean man came up to me and said, ‘You got big balls. I like them.’ Another time I saw a guy and a girl in the upstairs pool who were in this massive lip-lock and fooling around in the water. When the guy came out of the pool, he had the biggest erection I had ever seen.

“I think all the alcohol that they serve there makes people more sexual.”

City law states that “no establishment shall make facilities available for the purpose of sexual activities” and that “such facilities shall constitute a threat to the public health.”

Only days after Post inquiries, the city Health Department on Tuesday temporarily shut down all of Spa Castle’s pools. It cited the spa for “not following safety standards” and said the closure grew out of an investigation into a near-drowning on Feb. 19, when a 6-year-old girl’s hair became caught in an underwater vent. Spa Castle on Friday submitted a revised safety plan, which is ­under city review.

Messages and emails seeking comment from the spa and its owner on the allegations of sexual activity were not returned.

Between 2014 and 2015, the Health Department cited Spa Castle for 49 violations, including failure to post signs prohibiting discharge of bodily fluids in pools. In January 2015, a Manhattan outpost of Spa Castle, Premier 57 Spa, was shut down by health inspectors for operating without a license.

In December 2014, an 84-year-old spa-goer was found dead at the bottom of one of Spa Castle’s outdoor hot tubs. An autopsy revealed he died of hypertension and cardiovascular disease, even though family members insisted he was in good health.

“The Health Department has repeatedly inspected Spa Castle to assess compliance with NYC rules about bathing establishments,” said a department spokesman. “The department does not hesitate to take corrective actions, including closures, when an establishment is found breaking the law.”

The Post’s findings did not shock state Sen. Tony Avella (D-Bayside), who has been lobbying for years to revoke the spa’s license.

“I’m not surprised this type of thing is going on at Spa Castle,” he said. “Most of their problems stem from a lack of supervision.”

Additional reporting by Dean Balsamini