The Big Book of Alcoholics Anonymous describes the fellowship as “people who normally would not mix.” That’s a good way of describing James and me. I was 27 years old, a grad student, bored and curious — just like my ad said. James was in his mid-30s, a little too old and far too normal. He was not the kind of guy who’d approach me in another situation, at least that’s what I thought when I saw him. Then again, James and I would never meet in any situation other than this.

I was a Craigslist call girl. James was my first. I had gotten the idea from a friend. “There are ads,” she said, “placed by men, looking for” — she raised an eyebrow — “company.”

That night I got online. It was just as she’d described: SWM seeks non pro, GFE, a little fun. FS. DATY. BBBJ. A lady that speaks GREEK, possibly, a road of possibilities, a chance encounter, no strings attached. For 200 roses, 300 reasons, a generous donation, a happy ending. You can start any day that you like.

On the now-shuttered adult services section of Craigslist — to the left and below where you’d rent an apartment or sell a couch — you could find ads, written in their own coded language, from men and women and everything in between, all of them after one thing: the simple exchange of money for sex.

It was just what I needed. Working full-time as a research assistant at a hospital, I struggled to make ends meet. I was single for the first time in adulthood. Besides my ex, who’d been my high school sweetheart, I’d only slept with a handful of people. I shocked us both by calling off the engagement. I was not ready to start a family. I didn’t want to grow up. In the weeks and months after our breakup, I slept with anyone who’d have me — most of my male classmates and some of the women — until I’d alienated many of the people who had once been my friends. I was guilt-ridden. I was alone.

It was a Tuesday night after class, and I’d had three or four...