The adult entertainment industry is struggling to compete with free internet alternatives - and porn stars are having to get ever more resourceful, writes Louis Theroux.

On a movie set in an industrial area of Las Vegas, Tommy Gunn, one of America's top porn stars, was describing his ideal woman: "Self-sacrificing and caring and nurturing and wants to have children. Honestly, I'm not going to find her in this business."

In the eight years he's been working in porn, Tommy's done something like 1,200 scenes. Muscular, faintly Latin-looking, with a slight touch of Robert De Niro, he's built a reputation as dependable in an industry where reliability is a man's most highly-prized professional asset.

You can get some idea of the nature of Tommy's films from the titles. Addicted 2 Sin, Call of Booty, Fleshdance.

A few days earlier, at his rented ranch house in the countryside north of Los Angeles, Tommy had showed me the small army of statuettes he'd won for his performances - the porn equivalent of Oscars.

In his garage, amid the collection of motorbikes testifying to his past life as a mechanic, he'd taken down a few of his DVDs from a high shelf.

Find out more Louis Theroux - Twilight of the Porn Stars is on BBC Two at 22:00 BST on Sunday 10 June Or catch up later via iPlayer

Today's shoot was for a scene for an internet site. Tommy was playing a "secret shopper" checking out the customer service at a video store.

Tommy's "love interest", if that's the right phrase, was the store manager, played by 23-year-old Tasha Reign.

Tommy told me he'd been single for four years. He'd struggled to find a lasting relationship with someone who was willing to put up with his lifestyle - or possibly he found it difficult to love someone who was willing to put up with it.

"It's not normal leaving somebody you love to go and have sex with someone you don't," he said. Then, emphatically, he repeated: "That's not normal."

The job is tough at the best of times - and these are far from the best of times

To the millions of consumers who look at pornography, the lives of the male performers might seem in certain ways like a fantasy. Being paid to have sex with beautiful women five or six times a week? What's not to like?

For those who live the life, the reality is quite different. For one thing, the wages aren't much - $150 (£97) per scene. You can forget about pensions and health insurance.

Not to mention that the act of coition on demand, with lights on and cameras rolling, is not a skill every man is capable of. Or, given the shame and embarrassment and the toll it takes on one's relationships, would want to be capable of.

The job is tough at the best of times - and these are far from the best of times.

The porn industry is in crisis, its profits decimated by the impact of the illegal downloading of pirated content from YouTube-style sites and also amateurs uploading their own sex scenes on paid-for sites.

Fifteen years ago, when I did my first reporting on the porn business in Los Angeles, it was a very different picture. Back then, porn companies were making millions.

The big story then was the massive profits of a hidden industry that seemed as though it might be on the brink of being acknowledged by the mainstream.

A few porn performers were becoming celebrities: Traci Lords, Jenna Jameson, Ron Jeremy (aka "the hedgehog", because of his profuse body hair).

Image caption Free internet porn threatens DVD sales

One of the top male performers went by the porn name Jon Dough. So prized a performer was he that one of the high-end production companies, Vivid Video, put him on contract to work exclusively for them.

I interviewed him on-set around this time. He was starring in a remake of the adult "classic" Debbie Does Dallas, directed by an ex-performer called Paul Thomas.

Jon Dough killed himself nine years after that conversation, at the age of 43. Most of the industry put it down to the pressures of the business and the difficulties of making a living in a market that was saturated with free product. Several people blamed his death on declining DVD sales.

Jon Dough was married to a fellow performer, Monique DeMoan, who is now retired and living 800 miles from Los Angeles. She said her husband killed himself because of drug addictions.

Still, it says something about the industry that so many were ready link the suicide to the plight of DVDs.

The decline of the porn industry is part of a general trend affecting music, print journalism and mainstream movies. The many ways of getting content for free have slashed the profits of the professionals in their respective fields.

But where moviegoers and music fans may feel a loyalty to, say, Pixar or U2, and understand they need to pay for the fruits of their labour, the consumers of porn have less compunction about stealing the product. Many feel it's more moral not to pay for adult content.

Female performers have been resourceful about finding other outlets for their work.

Image caption Porn star Kagney Linn Karter boosts her income with webcam work

I spent a surreal evening at the home of a top porn performer, Kagney Linn Karter, while she did a live web show in her bedroom. Webcam work is one of the few kinds of content that can't be pirated, since it's live and interactive. While Kagney stripped on her bed in front of her laptop, I hid out in the kitchen with her boyfriend Monte.

Many female performers also work as prostitutes for extra cash. Where a female performer might make $600 to $800 (£388 to £518) for a straight sex scene in a movie, she can get double that - for less work - by "doing a private".

For many performers, the movies are now a sideline and a kind of advertising for their main business of prostitution.

While the wages stagnate, and the jobs dry up, the pressure on the performers continues.

During my visit, Monte expressed his unhappiness about a scene Kagney had just been booked for, involving a sex act so outlandish it can't really be described in a mainstream news forum.

For many performers, the movies are now a sideline and a kind of advertising for their main business of prostitution

The male performers' options are even more circumscribed. No prostitution for them, no webcam shows, and lower pay.

The top echelons of the profession, people like Tommy Gunn, still get regular work. But he still struggles with a sense of loneliness and the strange combination of stigma and fame that his very peculiar profession brings with it.

After his scene in Las Vegas wrapped, I joined him in his unglamorous motel on an unfashionable stretch of Las Vegas Boulevard.

I complimented him on the solid performance he'd just turned in. I was aware it was a strange thing to say, but I also wanted to acknowledge how much he'd had militating against him - the people, the length of the scene, the apparent lack of interest of his partner.

"That's my job," he said.

For a moment, there seemed something both sad but also oddly heroic in his ability to discharge the strange responsibility he'd taken on.

But for how much longer the job will exist is unclear.