Craig is a tall 16-year-old with a quiet but confident manner. He recently left his local comprehensive in a gritty part of south London with good GCSE results and wants to do an engineering apprenticeship after his A-levels.

He pulls a face when I ask if he or his friends have ever watched pornography. "I think as kids we found it funny. I don't think we watched it in the way it is supposed to be seen, you know, it was something to laugh at because it was outrageous," he says, clearly speaking in the past tense. "It's quite immature, isn't it?" Like most of his friends, he doesn't sext or arrange hook-ups using social media and has a long-term girlfriend.

Harry, 17, first saw pornography online with friends when he was "about 13" and reckons it is a normal part of teenage life. "Any teenage boy who tells you that he hasn't seen porn is lying. I've seen some crazy things online," he says. Harry doesn't currently have a girlfriend and he believes watching porn has affected how he thinks about girls and relationships. "It does change what you expect real girls to do and it changes the way you rate their bodies."

It is a warm late summer evening and I am at a youth centre in south London, a couple of miles from where I grew up. I am talking to teenage boys about their attitudes towards, and experiences of, sex and relationships. They chat about social media, online relationships, their thoughts around what consenting to sexual behaviour means. I think how complex their lives seem in comparison to my memories of being a teenager. Despite several years working as a youth mentor and then a counsellor in a variety of schools in London, I have plenty to learn about contemporary boys and sex.

For someone of my generation, growing up in the 1980s, the current accessibility of pornography is astonishing. I'm not sure how I would have dealt with the temptation of smartphones that make pornography instantly viewable all the time. In my single-sex secondary school there was widespread interest in pornography once boys hit puberty. X-rated videos, top-shelf magazines and "dirty" playing cards changed hands but they were not easy to find and were prized items. Interest in pornography generally declined as we got older and began dating girls.

Today boys don't even need to search out pornography. A survey for the BBC earlier this year revealed that only 22% of young people who had seen pornography were actually looking for it the first time they saw it online. I recently heard about a 10-year-old boy who was shown online pornography by a friend in his bedroom while his parents were downstairs. In March of this year, a 12-year-old boy pleaded guilty to raping his seven-year-old sister after watching pornography online. Blackburn youth court heard that it led to a desire "to try it out".

A 2013 report for the Office of the Children's Commissioner, called Basically … Porn is Everywhere, examined recent research on the impact of pornography on children and young people. According to the report, pornography "influences their attitudes towards relationships and sex; it is linked to risky behaviour such as having sex at a younger age; and there is a correlation between holding violent attitudes and accessing more violent media". But the report also said that we didn't know much about exactly what young people were watching and could not find conclusive evidence that pornography was now more extreme or hardcore than in the past.

The two main reasons boys give for watching porn, according to the report, are sexual gratification and passing it around among themselves. Girls are more likely to say that they watch pornography to learn about sex.

The availability of pornography is just one aspect of a highly sexualised culture aimed specifically at young people. Mainstream videos by the likes of Robin Thicke and Miley Cyrus look like soft-porn movies, advertising images feature an endless array of objectified bodies – there is little escape from erotic images in our everyday life. Surrounded by this constant stimulation yet frequently warned about the dangers of having sex, is it any surprise if young people feel they are getting mixed messages about what to do with their bodies?

Teenagers say that online porn is freely available. Photograph: Dan Chung for the Observer

Recent headlines about the sexual behaviour of boys can make for disturbing reading. We are familiar with stories about sexting (the sending and receiving of self-generated explicit images) and revenge porn (uploading sexual images or explicit videos of a former partner to social media sites). The police now warn teenagers that they face prosecution if they share such images, although in practice first-time offences are likely to be met with a caution. Earlier this year in Nottinghamshire a teenage girl who sent a topless picture of herself to her then boyfriend was investigated under the offence of having distributed an indecent image of a child. Her boyfriend, who forwarded the images to friends after they separated, received a caution.

Most of the boys I spoke to shared a naivety about the issues of consent and the law. "If a girl wants to send pictures to a guy then it is her risk. If she decides to send pictures to a boy, she is putting her trust in him and if the boy decides to expose her, then that is her mistake," says David, 18.

"Girls get a particularly bad deal with sexting," says Andy Phippen, professor of social responsibility in IT at Plymouth University. "Boys often don't reflect on the damage they can do by passing images on. Twenty years ago the kids you knew were the kids around the school gates, but now teenagers can find a much wider audience."

But he also worries about how boys are perceived. "I work with a lot of academics in this area, many of whom approach the topic from a feminist perspective, which automatically positions boys as 'bad' without exploring why they have the views and attitudes they have and what it is like to be a boy growing up in this society," he says. "The vast majority of young people just want a nice boyfriend or girlfriend, but they are living in a highly sexualised society that makes things very confusing."

Tom Sherrington is passionate about the importance of educating boys about sex and relationships. As the headteacher of King Edward VI grammar school in Chelmsford, he spent several years teaching year 8 boys (12- and 13-year-olds) sex and relationship education (SRE) and he has written a series of blogs for the Guardian about his teaching methods. He left the school in the summer to become headteacher of a comprehensive in north London.

"A high proportion of kids get poor sex education. There are not enough people who are able to talk about sex in a complex way. Of course, you want to talk about morals and values and advocate sex as part of a stable and loving relationship. But some boys are going to have sex in a one-night stand. You want to promote what you think is the ideal situation, as well as acknowledging that other things happen," he says.

Sherrington thinks it is easy to spot the boys who have seen pornography at an early age. "Their level of awareness of different types of sex acts is quite high. But often these boys can't relate to the porn they've seen. [Through talking about sex] they are quite relieved to discover the other aspects of sex, such as intimacy and kissing. The majority of boys don't want to be like porn stars and they are pleased to learn they don't have to emulate that."

Poor-quality sex education is a common complaint among experts: too much time spent putting condoms on bananas and not enough teaching children about healthy relationships and debunking myths about what is normal sexual behaviour. In 2013, Ofsted stated that 40% of sex education in schools was inadequate. Channel 4 recently announced that it is going to launch a new programme called Sex in Class alongside a Jamie Oliver-style campaign to introduce a GCSE in sex education. The programme will be presented by Goedele Liekens, a Belgian sex therapist and UN goodwill ambassador, who believes sex education in British schools is "hopelessly out of date" and should start at an earlier age.

Craig says: "In year 8 we were just told about what happens in the worst-case scenarios. It wasn't so much about sex, it was what happens if you get a girl pregnant. I think they were trying to warn us off [having sex] and we only had the lessons for a week."

Male expectations of what can happen during sex, especially what girls should "do" sexually, have been blamed for the rise in sexting and for putting pressure on young women to perform certain acts. In July, BMJ Open (a sister publication to the British Medical Journal) published an article that aimed to "explore expectations, experiences and circumstances of anal sex among young people".

The study was carried out by researchers at the London School of Hygiene and Tropical Medicine who interviewed 130 male and female teenagers aged between 16 and 18 in heterosexual relationships. They discovered that anal sex was often "painful, risky and coercive for young women". Pornography was frequently cited as an explanation for engaging in anal sex. The report also found that "many men said they encouraged one another to try the practice, and men and women said men wanted to tell their friends that they had had anal sex".

One of the key recommendations in Basically … Porn is Everywhere was that sex and relationships education should be renamed relationships and sex education to signify a change in emphasis in what is taught. "Most boys do not talk about sex with their families. They will talk within their peer group, but sex education lessons provide an opportunity to talk about the issues in a different way," says Sherrington.

Many boys don't feel that they have a sympathetic adult to talk to about relationships. Although children of primary school age are generally able to talk to a parent or teacher, teenagers, especially boys, worry about being judged or blamed. When I ask Jamal, aged 18, if he would ever talk to his friends or parents about relationship problems, he sounds amazed. "No, I would never do that. It would just be too weird, I don't feel like that is something I would feel comfortable doing. I'd rather keep it to myself."

"Women talk more freely about this stuff – with boys there is a lot more bravado, a lot more bluff," says Luke Carter from Respect Yourself, an innovative sexual health campaign based in Warwickshire. "Coventry University recently did a study about young people's attitudes towards accessing sexual health services and young women were much more likely to use them because they have a better attitude towards getting help."

Porn has changed how teenage boys view relationships and women, critics say. Photograph: Martyn Vickery/Alamy

The Respect Yourself website is one of the more interesting attempts to demystify sex for young people, and dispenses chatty, informative and non-judgmental advice. Typical questions include: "I come too quickly; what do I do?"; and "I have a massive crush on my female music teacher and spend half the night masturbating. Please help as I want more time with my teacher and I need a lot more sleep" (the latter written by a 12-year-old girl).

Based on a "sex-positive" approach, which places the emphasis on embracing your sexuality while stressing the importance of safe sex and consent, Respect Yourself uses ideas from the Dutch model of how young people should be told about sex, which have helped lower the Netherlands' national rate of teenage pregnancy and sexually transmitted infections.

At Respect Yourself, they noticed something that Tom Sherrington also found in Essex. Given the right encouragement, teenage boys are keen to tackle thorny issues such as misogyny and consent, and want to develop the emotional skills to help them handle their relationships.

"If someone says, 'Oh, that girl is a slag', we use it as an opportunity to think about the attitudes behind the words," says Carter. "'How would you feel if that was your sister being called a slag?', then you are able to unpick things a bit more. Young guys don't like to be preached at but they will listen if you pitch things at the right level."

"When I talk about consent it is often in the context of what does it mean to be in a relationship. How do you move from friends to girlfriend and boyfriend? Teenagers don't usually sit around saying, 'Shall we have sex now?'," says Sherrington. "I'm not giving them a set of rules, but trying to give them a sense of personal empowerment about their decision-making."

The young men I met for this article were surprisingly open when talking one-to-one about sensitive issues. In their different ways, they were all coming to terms with how they should behave as sexually active adults. Professor Phippen says: "Boys are desperate to discuss these issues. When I go to workshops for young people the first five minutes are on porn, then they move on to body image, respect and relationships.

"In one SRE workshop the boys and girls were divided and asked to write questions for the other sex," he continues. "The girls wanted to know why boys 'always wanted them to wear makeup and revealing clothes', but when it came to the boys they wanted to know why the girls 'always wore makeup and revealing clothes'."

The names of all the boys interviewed have been changed

What men wish they had known about sex

Playwright, 43

What I wish I'd known…

That it is often very funny and sometimes afterwards you can feel so happy you just laugh for ages. That there is no normal way of having sex but that it is built on communication. That it is different every single time. That most boys don't have sex until they're about 18. That most people don't have great sex like on stupid porn movies every single day five times a day. That going down on girls is such great fun.

What advice I'd give…

Know that there is no orthodoxy. Know that it's not a competitive sport. Take responsibility for protection. Having sex is one of the most intimate things you can do with another human being. Your job isn't to fuck like a porn star but to be brave enough to tell the truth and to ask the people you're having sex with to be as truthful as they can. That this is never easy. Use lubrication. Keep your eyes open.

Gavin Turk

Artist, 47

What I wish I'd known…

By the time that I was 15 I had more or less tried everything there was to try. Before that I didn't know that much about it – which I think was a good thing. Nowadays there's so much more access to sexually explicit material in all sorts of forms, including commercial pornography. People are being sold stuff using tits and arse and if you buy this product it will make you sexually potent, when in fact it's probably the opposite. I think that all the media innuendo can be unhelpful and make for an unhealthy relationship with sex.

What advice I'd give…

There are many secrets related to sex and it is often very difficult to articulate them – language all too often lets us down. I do think that it is important to connect affection with sex, I don't necessarily think that you have to be in love with someone to have sex with them, but there should be respect and trust. Beyond that, get on with it and try to do as much as you can before you find somebody that you want to be with forever. But please, try not to get VD.

AN Wilson

Writer, 63

What I wish I'd known …

The thing about sexual feeling – I've found – is that, like one's feelings when friends die, it is always a surprise. You think – "Gosh!" I don't know any more about sex now than I did when I was 13. I do not think it is something human beings, or other mammals, "know about". Reading feeds one's thoughts – if not one's knowledge, exactly. In my mid-teens, I thought DH Lawrence was the last word. In middle age, John Cowper Powys accurately conveys the way in which sexual feeling operates in life.

What advice I'd give …

I would not presume to give advice to anyone about sex, especially not teenagers, who probably have a far more varied experience than myself. Most intelligent teenagers would presumably share my horror at the existence of internet porn. It did not exist in my youth. As a shy youth, I could easily have become addicted to it, and it must be mental poison that drives you mad. ,