Fuck for Forest use porn to spread their green message. In a new documentary, they are taken to the Amazon to meet a threatened Peruvian tribe. But how can they rescue the planet when they can barely look after each other?

"Blood and sperm. The perfect mix," says a tattooed hippy, as he licks both off his hands, having just had sex with a woman in front of a small audience in a Berlin basement. "Life-giving fluids we are all so afraid of. We're so afraid of ourselves! It's all organic." It's not everyone's idea of popular entertainment, but this scene can be experienced at a safe distance in a new documentary, F*ck for Forest, detailing the activities of the group of the same name (without the asterisk). They enjoy confronting society with sex, nudity and bodily fluids, but what Fuck for Forest (FFF) really want to do is save the world. So this isn't just pervy performance art; it's also fundraising.

Few people would imagine any overlap exists between pornography and environmentalism, but FFF smash the two concepts together right there in their brutally blunt name. It's a concise signifier of what they do and how little they care about what you think of it. The live displays are a sideline; funds are primarily raised via their website, which has images and videos of its core staff members and whatever volunteers they pick up on the street in myriad sexual permutations, from naked people up trees to chaotic orgies. Subscribers pay about £10 a month, and the proceeds go towards rainforest conservation projects in South America.

It's difficult to know how to categorise such an enterprise. Is it kinky eco-activism? Porn for foliage fetishists? Exhibitionism with the fig-leaf of a good cause? FFF have a better question: What is more obscene, they ask, the depiction of people enjoying their sexuality or the destruction of our natural environment?

"Sex is often shown to attract us to buy all kinds of bullshit products and ideas, so why not for a good cause?," says Tommy Hol Ellingsen, FFF's Norwegian co-founder. "The human body is considered more offensive and threatening than most things in the industrial world around us, like cars, but I don't see the naked body in itself as a threat to the morals or values of modern society. I think it's more a mass psychosis people have. Why we are destroying the planet may be somehow connected to the values modern humans have created for themselves."

Tommy and his Swedish partner Leona Johansson can talk at great length about the ills of western society, freedom of expression, the sanctity of nature and nobility of indigenous tribal life, but in the documentary their philosophy is put to the test. The first half details their eco-hippy existence, wandering the streets of Berlin, propositioning strangers to contribute to the website, getting stoned, having sex, and subjecting audiences to their performance art (if the "blood and sperm" part sounds shocking, wait for their terrible folk songs). But then FFF's dreams are confronted with reality, in the form of a journey to their much-idolised Amazon rainforest, at the request of a threatened Peruvian tribe. It would spoil things to reveal what happens when they get there, but let's just say it's not quite the tribal connection they hoped for.

"I really don't want to judge them, but my first reaction was, they're trying to save people on the other side of the planet when they can hardly help each other," says Michal Marczak, director of the documentary, who spent seven months filming FFF in action. "They live in a little fairytale wonderland, according to their own rules. They never plan anything, even what they're going to do the next day. There are no rules. That's what intrigued me about them."

But Tommy and Leona dispute the impression Marczak's film creates of them as naive hedonists. From a cheap guest house in Mexico, where they have been attending a "worldwide rainforest gathering", they explain how the encounter was set up by the film-makers. "They pulled strings to get us to that specific tribe," says Tommy. "For eight years, Fuck for Forest has already been working with different native people all over South and Central America, and we know a lot about the conflicts there. If we had organised that trip we would have known a lot more about that group and specifically what they needed help with, but when we got there, nobody knew anything about us. We wouldn't have ended up in that situation if it wasn't in some way manipulated by the movie directors."

FFF does in fact, have a solid record when it comes to the "forest" part of their equation. While accounting may not look like their strong suit, Leona estimates that the non-profit organisation has made in the region of €100,000 (£85,000) a year, since it started in 2004. Their website details how the money has been spent on buying up land and promoting permaculture and indigenous lifestyles in Brazil, Peru and other countries. And as the film attests, they also live a frugal lifestyle, wearing clothes and eating food they find in rubbish bins, rather than spending the charity's money.

Marczak admits that he facilitated the expedition seen in the film, but he never set out to prejudice viewers against FFF, he says. In fact, compared to other NGOs he spoke to in the Amazon, their methods are relatively effective: "A lot of aid workers there seemed to have a kind of post-traumatic stress disorder, where they come in full of hope and full of western money, then after a couple of years they feel depressed that there's not a lot they can do to help. In some cases, 80% of their money gets lost to middlemen before it reaches the people they're trying to help. Their idea is to work with little eco projects where they can oversee them and give money directly. In a sense, they're doing it the right way."

FFF's method of directly funding small projects isn't entirely down to strategy. It's also because most major aid organisations have rejected them. Both the Rainforest Foundation and the Norwegian World Wildlife Fund turned down their donations once they discovered what they did, claiming that other donors would disapprove of their fundraising methods.

It's no wonder FFF feel such affinity with isolated tribes on the other side of the world; they don't seem to have many friends, and at times they seem determined not to make any. When Tommy and Leona first started the website in Oslo, they received a grant from the Norwegian government – a decision the authorities regretted when Tommy and Leona caused a stir by having sex on stage at a Norwegian music festival later that year, while a hardcore band called the Cumshots played along. That led to an obscenity trial, at which Tommy pleaded for the cause of public nudity and dropped his trousers in court. Shortly after, they relocated to Berlin, where they've continued to make enemies. In 2009 they were ejected from an anarchist congress in the city for insisting on the right to remove their clothes during a workshop entitled "Anarchy and Sex". The controversy resulted in the entire congress being shut down early. In 2011, they took things even further by interrupting the Ascension Day service in Oslo Cathedral with a naked protest (in defence of a priest who was sacked for writing about sex). Shocked members of the clergy had to drag them off the altar. For advocates of sexual freedom, they sure rub people up the wrong way.

If FFF's rose-tinted ideology and confrontational stunts haven't made them enough enemies, there's also the fact that they deal in pornography. ut few of the complaints traditionally levelled at the porn industry really apply to FFF. There are no airbrushed, hairless, cosmetically augmented "porn star" types to be found on their pages. The "models" are regular members of the public. There's no policy towards gender, body type, sexual orientation or disability, although the majority of their 1,300-odd contributors seem to be healthy, young and white. The people decide what they want to do. Nobody is paid. You could call it Fair Trade porn.

"My first impression was, who the hell would ever watch this?" says Marczak. "And even if they would, who would pay for it? It's really vulgar and its very … hairy. Nobody shaves their armpits, and it's really badly lit. But I noticed that the people mostly seem happy in it. There are moments when they just left camera on for little while after they've finished and you see genuine emotion in people, like you hardly ever see in porn films." As a film-maker, Marczak grimaces at FFF's amateurish methods, but the amateurism is entirely deliberate, says Leona: "Usually the porn industry treats sex more like a product, but we have a more impulsive relationship with sex and nudity. Sex, for us, is something that works best when you're not thinking too much. The main idea is to have fun, not to make a product. We don't stage anything. It's all based on what people are willing to do for us.They don't even use the term 'pornography', she says. "It's more like a documentary, or a nature programme about human sexuality in the modern world."

That put the makers of F*ck for Forest in a tricky position. "Whenever they were filming, we were waiting to film, and whenever they stopped, we started," says Marczak. "And there were moments when we were filming at the same time and I was asking myself, 'Wait a minute. Am I missing something?'" You could possibly argue that F*ck for Forest, the movie, is only slightly less pornographic than FFF themselves. But the film views the raw sexuality of the real pornographers through the filter of professional documentary presentation. Blood and sperm are kept at a safe remove. It's sex with a condom.

The movie also exposes the ethical and moral jungle of modern society, where it's not necessarily straightforward to gauge what's right and wrong, and it does so entertainingly, at times hilariously. FFF aren't all that happy with the documentary but nor are they particularly bothered by it. They're into freedom of expression, after all. And as Marczak observes, even negative publicity about the group tends to drive up their recruiting. What annoys FFF more is the suggestion that they live lives of carefree indulgence. "That's bullshit," Tommy says. "We have so much responsibility. We have so much to do with these projects. It's a really heavy subject to work with sexual repression and ecology. With the website, all the uploading and emailing, we're a small group keeping it together and it's an incredible amount of work. But we're subject to so much suspicion, and we have to answer for so much of what we're doing. We're giving so much of ourselves to this."

• F*ck for Forest is available now on iTunes and in cinemas in the UK from 19 April