Many teenagers regularly view porn. But does this give them a distorted view of sex and relationships, and is there anything that can be done?
For many parents, the idea of their child seeking out, or stumbling across, online porn is the stuff of nightmares.
But with many children's bedrooms equipped with computers and the proliferation of the smartphone there are plenty of parents who would accept the inevitability.
There is no clear statistical picture as to how many children access porn online, or how often they do it.
In 2011, an EU-wide survey found that a quarter of 9-16 year-olds had seen sexual images and only 11% on websites.
Almost a third of 16-18-year-olds have seen sexual pictures on mobile phones at school at least a few times a month, a 2010 YouGov survey suggested.
The National Association of Head Teachers is calling for children to be taught, "in an age-appropriate way", about the impact of pornography as part of the national curriculum. So from a young age, about 10, children would be taught about internet safety and warned about content, while teenagers would cover the issues in more detail.
"Children are growing up in an overtly sexualised world and part of this includes easy access to pornography on the internet and they need the skills to deal with that," says policy adviser Sion Humphreys.
The key concern is that teenagers personal lives, and even their adult sex lives, will be shaped by what they have seen.
Porn is not normal sex, the campaigners note.
Cindy Gallop, an advertising executive turned web entrepreneur, has set up a site which compares the "porn world" with the "real world" of sex.
Gallop, who spoke on the subject at a TED conference in 2009, says this "ubiquity, and freedom of access to online porn, combined with a society that is reluctant to talk about sex", has resulted in "porn becoming the default sex education".
One of the campaigners' concerns is that teenage girls and boys feeling pressured to do certain things they would not otherwise choose to do.
The idea is that if many teenagers are watching porn, and a certain activity is widespread in porn, their inference is that it is widespread in people's sex lives.
Commentators have already noted how aesthetic standards spread from the world of porn. "Brazilian"-style waxing is now considered normal by many in the US and UK. Even teenagers can feel they have to conform.
Boris Johnson's sister, Rachel, made headlines when she revealed her horror at her 15-year-old daughter's desire to wax.
In 2010, a Home Office report warned the "drip-drip" exposure to sexual imagery - which included pornography, "lads' mags" and sexual imagery in advertising - was distorting young people's perceptions of themselves, encouraging boys to become fixated on being macho and dominant, and girls to present themselves as sexually available and permissive.
One 17-year-old, Rebecca, says porn changes boys' expectations of how girls should look. "Long hair, big boobs, big bum. If I had short hair, guys would be like, why short hair? You should grow it out."
Her classmate Femi says porn can worry boys too.
"Maybe you're not physically living up to what porn is showing you," he says.
Karen, 20, says when she was 16, her boyfriend and his friends watched online porn "like it was a hobby". She says he would often watch it in front of her, copying what he saw.
"I thought there was something wrong with me for not enjoying it," she says.
A survey of 16-24 year olds by the University of Plymouth and the UK Safer Internet Centre found that one in three admitted porn had affected their relationships. ChildLine said it had seen a 34% rise in the last year in the number of calls from teenagers distressed by sexual images they had viewed online.
But conclusive proof of sexual behaviour changing among teenagers is hard to come by.
At the moment, personal, social, health and economic (PSHE) education - which can include teaching on sex and relationships - is not compulsory in England, unlike other parts of the UK, although it is in the national curriculum.
Secondary school science lessons include basic biology. Beyond that it's up to schools how to address sex, and parents have the right to withdraw their children from any sex education.
So should school children be taught that pornography is not "real"?
Leonie Hodge, from the charity Family Lives - which has taught more than 7,000 students about the subject - is a firm believer that children need to learn the difference between porn and reality.
With 90% of children owning a smartphone, she says it is no longer relevant to talk about "making a baby".
"Teenagers are bombarded with pornography from a young age, you can't escape it. Its patronising to say they can't cope with the lesson because they can."
The charity uses role-play exercises to discuss with young people how they would react if they receive indecent images, and what porn makes them feel like.
But the National Union of Teachers say referring to pornography in lessons is a step too far and that it should only be discussed if students approach it.
This is the "most wired generation" ever, says Gallop.
And the 52-year-old, who has spoken of her series of relationships with men in their 20s, says her first-hand experience includes young men who derive many of their sexual attitudes from pornography, rather than anything more loving or intimate.
"I also get emails every day from young people pouring their hearts out, saying they had been utterly confused, and had no idea what was normal," she says.
But as well as teaching children about the impact of porn, Gallop thinks parents should also have more of an open dialogue with their children.
"The key is not to get embarrassed, or say something like 'nice girls don't do that', and it doesn't matter if a child doesn't really want to listen, the important thing is to keep the line of communication open," she says.
Siobhan Freegard, co-founder of Netmums, says the issue of online porn regularly comes up on forums, and there is quite a strong feeling among mothers that protecting their child from it, or educating them about it, is a parent's responsibility.
But she says mothers frequently panic when they come across porn on a computer at home.
"It can be a minefield - many don't know what to do or say. For example a single mother may struggle with teenage boys, a single father may not know how to approach the subject with his daughter. In very traditional households, they might not even talk about sex at all.
"The ideal solution is for schools and parents to work together," she says.