Something strange kept happening to Chrissy Monroe as she strolled along Madison Avenue.

“I’d be walking down the street and some guy would stop and offer to buy me something,” said the Upper East Sider, who is the CEO of the Millennial Plastic Surgery clinic in Midtown.

Gifts were the good part. But Monroe said that once she went on dates with these serious, wealthy, powerful men — often captains of the boardroom — they would turn to mush the second they stepped into the bedroom.

“I’d be with these very bossy men who were CEOs, surgeons or judges. But in bed they’d become completely different,” Monroe explained. “They’d want to lick my feet. They’d want to paint my toenails. I had one guy who would always want to come and clean my house. I’ve always attracted these . . . guys that want to worship my feet and treat me like a goddess.”

So the 46-year-old decided to make the most of it, and give these men what they want.

Like dozens of other ladies hungry to hook up with the billionaires who haunt the Upper East Side, Tribeca and other moneyed parts of town, Monroe has turned to Betsy Cox, the owner of Blackbook Concierge. A couple of months ago, Cox launched a one-of-kind class for women with a particular problem: understanding the sexual fetishes of the commanding men they sleep with.

She said that BDSM attracts so many formidable types because “the amount of control these men have is astronomical.” In other words, if you are surrounded by yes men and people greenlighting your ideas — if you dominate all day long — it can be a turn-on to have someone resist you.

“They’re looking for somebody to tell them what to do for once,” explained Cox, 55.

Even alpha males, it seems, need a break.

In her Soho loft, Cox hosts classes involving five well-heeled women and a dominatrix. The four sessions cost $2,000, total, and cover everything from the ABCs of BDSM (bondage and domination, sadism and masochism) to psychological techniques women can use in their love lives, e.g. “how to get into a man’s head” — and how to treat social, romantic and professional interactions like a game between a dom and a submissive.

Jennifer Gaydos, 36, signed up to learn more about the kinks of the jet-setters she meets on the invitation-only dating app Raya.

“It’s not like you are there to learn how to give a blow job,” she said. “It’s more psychological. What makes a [rich man] want to relinquish control?”

The buzzy Showtime series “Billions,” about hedge-fund tycoons and their debauched antics, has also exposed the once-secret BDSM culture among New York City’s rich and powerful. On the show, Paul Giamatti’s US Attorney character works hard then comes home to be dominated by his therapist wife, played by Maggie Siff in a killer pair of knee-high stiletto boots.

“All of my friends read ‘50 Shades of Grey,’ ” said an anonymous recent Cox graduate of the 8-year-old S&M novel that became an international bestseller. “Then ‘Billions’ brought it back to the conversation.”

Cox said she started arranging the classes after a friend expressed interest in upping her dating game by learning how to “dom.” She mentioned it to other pals and received immediate interest.

“It’s so competitive to find a man,” Cox added. “Some of these women can have anything but they can’t get a man. By learning dominance and understanding what drives these men, women are getting an edge.”

News of the course is shared only by word of mouth, but there are already 30 women on the waiting list, Cox says. Because her clients summer in Europe, she’s now looking to host classes in Gstaad and St Tropez. She hopes to make “domming the new pole dancing.”

That makes sense to student ­Gaydos.

“It was all upper-class women, everyone from CEOs to private-school moms,” the founder of ­Gianella Swimwear said of the class she attended. “I think we were all kind of shocked. No one said anything at first. The mistress showed us bonds, whips and devices to put on men’s private parts.”

But Goddess Adrienne, the dominatrix who leads the classes at Cox’s loft, says it’s not just about the accessories.

“BDSM is misunderstood. We want to give women confidence that they can bring it into the bedroom or the boardroom,” she said. “The common denominator among the women who take my class is that they are dating in New York. They want to be loved and appreciated. But New York is a hard place. It helps to understand the tendencies of these guys.”

Adrienne, who has been a professional dominatrix for five years and charges between $500 and $1,000 an hour, describes herself as a “true sadist” whose clients are mostly rich men. Her most recent session before speaking to The Post was with a hedge-fund manager.

“One of my favorite fetishes has always been ball busting, which is literally when you kick someone in the balls,” she said. “I get the hugest giggle out of it. My ­clients always want more.” (She draws the line at changing diapers for “adult babies.”)

The anonymous recent graduate said she was surprised the first time a man asked her to choke him during sex: “I didn’t know what to do.”

But after taking the BDSM classes, she says she looks at dating and sex differently — and that it’s as much about empowering herself as dominating her partners.

“I won’t hook up with a guy now unless he is giving me what I want, because what’s the point?” she said. “I feel like girls [will sometimes] have sex with a guy even if they know it won’t be good sex. Now I know that I am in charge of what I want, it’s ­really empowering.”

Monroe confirmed that learning to dom has already paid off in her love life.

“I am seeing this guy and he’s a catch,” she said. “He is very successful. He is powerful and in the public eye. But he is already giving me a submissive vibe. Now I know how to handle him and how to play the psychological game when we take it to the next level.”