A nice dinner at an atmospheric Lower East Side restaurant concludes with my cool, demure, diminutive girlfriend pulling out a wad of bills that could choke a donkey. “I’ve got this one,” she says, peeling off a hundred and securing it to the table with her wine glass. “Shall we?” Kate is putting herself through grad school with student loans and clerical work; more often than not, I pay for dinner. Tonight, however, is the third time she’s had sex for money, and the third time she’s taken me out to eat afterward. It’s become a sort of tradition.

Twenty-six-year old Kate and I have been dating, non-exclusively, for more than a year. She had planned for her two-year stint in New York to be a time of romantic and sexual exploration with a rotating cast of interesting characters. I was lucky enough to land the only recurring role six weeks into her first semester.

Kate had been in the city for about seven months when she decided to put an ad on Seeking Arrangement, a website that exists to connect well-heeled men with actresses, models, students (750 from New York University alone, the company claims) and, according to an internal survey, thousands of public-school teachers. The “sugar daddy” and “sugar baby” agree how the former will compensate the latter for companionship—however that is defined. Sometimes, the sugar daddy will shower his benefactee with gifts, pay her rent or devise another setup that allows one or both parties to distance themselves from prostitution and its stigma.

But Kate doesn’t shy away from that fact that it’s sex work; she prefers cash. Only a tiny portion of what she earns gets splashed out on a mid-priced meal like ours, though. The remainder is put into a college fund for her nieces and nephews.

I had an uneasy feeling about her entering the skin trade, but part of what had attracted me to Kate was her boundary-pushing sexuality.

“I’m only going to be with people I’d consider having sex with anyway,” she said after posting her “sugar baby” profile and sorting through the numerous responses. Mutual attraction is a very new development for the world’s oldest profession, a real boon for women who seek to convert sexual capital into money ad hoc.

Of the many offers that came flooding in, it was a good-looking, recently divorced 48-year-old surgeon that fit the bill. Over lunch at a trendy restaurant, he offered Kate $600 for an hour of her time one afternoon that week. (Typically, sugar daddies and sugar babies settle into a more rolling retainer agreement, which again distinguishes it from old-school sex work.)

“Are you going to do it?” I asked. Part of me imagined that the meeting alone had sated her curiosity, but I was wrong. Kate said that she found John slightly odd—but not quite odd enough to forgo what she hoped would be an easy payday. I had an uneasy feeling about her entering the skin trade, but part of what had attracted me to Kate was her boundary-pushing sexuality. I relished the opportunity to share in one of her adventures, albeit at some remove. My role was to know exactly where Kate was going and when, as a safety measure; we also planned to meet immediately after her “date”.

Going in. My stomach sank as I read her two-word text shortly before the 3pm appointment. I knew from experience that the outcome could go any number of ways. In 2010, Jen, an Ivy League grad I’d been on a couple of dates with, began seeing a Brazilian banker through Seeking Arrangement. He paid her rent, took her to the city’s most celebrated restaurants and jetted her around the world with him. They started having so much fun that often she didn’t have the time or need for her day job. Within six months, she and Beto had fallen in love and moved to Sao Paolo, where they plan to start a family.

Would Kate go that route? Or might she come through my door in tears, having chased her curiosity too far? Although she was the epitome of level-headed, actually having sex for money seemed like a huge departure from her normal brand of sexual curiosity.

Kate arrived at my apartment less than 50 minutes after her session began. With her lipstick smeared, she fanned the bills in her hand and playfully waved them in my face, giggling. “Are you okay?” I asked. “Of course,” she said with characteristic sang-froid, smelling faintly of aftershave.

Pervert that I am, we were naked within a minute of her coming through the door. Amid particularly frenetic action, she gave me the broad strokes. They’d spoken for five minutes, had intense sex for another five, then he spooned her for a further ten. All in all, John had been respectful, courteous, even tender. After just 20 minutes, she left. Pro-rated, her hourly fee was $1,800.

On the walk back to my place, Kate decided that instead of seeing John on an ongoing basis, she’d visit him when she felt like it. (Her unavailability caused John to nearly double his fee by the next date.) She also decided that one guy willing to pay her for sex was plenty.

Kate, like Jen, is a new type of prostitute: One who is living out a sexual fantasy in exchange for money that she could certainly use but doesn’t need. With more than a million members—the vast majority of them female sugar babies—Seeking Arrangements has pushed these mutually beneficial relationships into the realm of acceptability.

I’ve only told a few friends about Kate’s newest adventure. They all asked if I thought our relationship was devalued by her decision to exchange sex for money. It hasn’t been. In fact, I think it’s strengthened it. Being the only person to know about Kate’s adventures makes me her confidant, the beneficiary of her trust. I also know that if I could command $1,000 for a 20-minute date with someone I’d have sex with for free, I would jump at the chance.