John and Allie, the Los Angeles-based producers and hosts of the podcast Swingercast, describe themselves as “a happily married couple having the time of our lives”. Swingercast, which sees the couple document their swinging experiences, is a typical example of the boom in erotic podcasts. An area of interest that has translated from the book market, where the Romance Writers of America reported in 2013 that sales of romantic novels amounted to $1.08bn (£835m). Between 2010 and 2015, 39.8m physical erotica and romance books were sold in the UK alone. Add to this the rise in erotic e-books (EL James’s Fifty Shades of Grey started out as an e-book) – the popularity of which might be down to readers’ preference to be discreet when indulging their passions while commuting.

Though romantic books have been around for a long time – ancient Greece had romantic tales, and what we now think of as the popular romance novel appeared as far back as the 18th and 19th centuries – erotica fans are now slipping on the headphones to get their thrills. There’s a wide spectrum of erotic podcasts – sexy, educational, philosophical – with different focuses. Bawdy Storytelling, for instance, features real-life sexual stories from listeners; the more straight-laced advice pod, Savage Lovecast (disappointing, given the name); the “anti-slut shaming podcast”, Guys We F****d, with one episode featuring Amanda Knox, of all people.

Bawdy Storytelling has been going for 12 years, but now its live shows are recorded for its podcast, and filmed for YouTube, too. People submit their stories online, and “sexual folklorist” Dixie de la Tour picks the ones to be performed in shows across the US.

De La Tour tells me that the stories she looks for must be related to “sex, kink or gender”, and then the shows are themed. Previous examples include: Bottoms Up: True Stories of Anal and NerdGasm. She has noticed the rise in erotic podcasts; it makes sense, she tells me. “I started with trying to let one person at a time experience the sexual underground. Then storytelling allowed me to give access to hundreds at a time. Podcasting means that millions can ‘attend’ a live show.”

In the UK, My Dad Wrote a Porno, hosted by Jamie Morton, whose dad literally wrote a porno, sees Morton read extracts of his dad’s porn series (called Belinda Blinked), with co-hosts and university friends Alice Levine and James Cooper providing warm, witty commentary. My Dad Wrote a Porno is hugely popular. It has, Morton tells me, more than 120m downloads. A live show has toured 11 countries, playing to audiences of 70,000 and, currently, the team are finishing dates in Copenhagen. Celebrity guests have included Stephen Mangan and George Ezra.

“I don’t think we could ever have imagined how stratospheric [the podcast] has gone”, Morton says. “We knew straight away that the source material was really special – we believed that if my dad’s writing had made us cry with laughter, that there was a good chance that other people out there would enjoy it, too.”

The team behind My Dad Wrote a Porno

Morton’s mum was reticent at first. “She definitely wasn’t over the moon about the prospect at the beginning,” he says. “But we would never have moved forward with it if she, or my sisters, weren’t fully on board. I remember her refusing to listen to the pilot and her traipsing upstairs in a sulk, with a bottle of wine in her hands.”

She is fine with it now, and understandably: there’s an HBO comedy special coming in 2019. Meanwhile, the Belinda Blinked books themselves have their own fans, with readers gobbling up the e-books and discussing them in dedicated book clubs.

This October sees the launch of Stitcher’s Purple Panties podcast. Purple Panties, the 2018 anthology of lesbian erotica curated by Zane, a New York Times bestselling author (and, um, tax avoider), offers a sexual take for women and non-binary people of colour, a community that has often been sidelined in traditional erotica and romance content. The “erotic noir” anthology, which was written by women around the world, promised to “blow the sheets off beds everywhere”.

Now, the book (which is already available in audiobook form) will hit kinky ears in a fiction podcast. It will follow the lives of protagonists Maddox, Loren and Stephanie, who “go against the grain when it comes to sex”. Given that the pod is only available to Stitcher subscribers, Purple Panties might not reach the widespread levels of other pods, but Zane has a dedicated following and has written more than 30 books.

There doesn’t seem to be any sign of the erotic podcast boom slowing: podcasts of all types remain an increasingly popular medium, and sex will always sell. Consumers are turning to more subtle ways to get their kicks, as the mainstream porn industry attracts more and more scrutiny (though PornHub still clocks up 16m views a month). In particular, some studies have suggested that brain differences between men and women mean that women respond better to audio and written stimulation. But the reach of certain pods, and the range of niche interests, suggest it isn’t just women listening. Amazon’s Audible.com has a list of its top 100 erotica audio books. There’s even Potterotica, an erotic podcast for Harry Potter fans. The latest episode is the brilliantly titled Hokey Poke Me.

So, it’s up and up for erotic and sex-themed podcasts and audiobooks – just make sure you definitely have your headphones in when you’re listening on the train, so you don’t end up broadcasting an orgasm to everyone in the quiet carriage.