Sometimes you hear a story that is so awful that it refuses to leave your mind, no matter how fervently you beg it to go away.

I was told one such story recently by a family doctor. Readers of a squeamish disposition may want to look away now.

I was having dinner with a group of women when the conversation moved onto how we could raise happy, well-balanced sons and daughters who are capable of forming meaningful relationships in an age when internet pornography is as freely available as a glass of water. Porn has changed the landscape of adolescence beyond all recognition. Like other parents of our generation, we were on a journey without maps or lights, although the instinct to protect our children from the darkness was overwhelming.

Boys and girls can fall victim to 'sextortion'

A couple of the women present said that they had forced themselves to have toe-curlingly embarrassing conversations with their teenagers on the subject. “I want my son to know that, despite what he might see on his laptop, there are things you don’t expect a girl to do on a first date, or a fifth date, or probably never,” said Jo.

A GP, let’s call her Sue, said: “I’m afraid things are much worse than people suspect.” In recent years, Sue had treated growing numbers of teenage girls with internal injuries caused by frequent anal sex; not, as Sue found out, because she wanted to, or because she enjoyed it – on the contrary – but because a boy expected her to. “I’ll spare you the gruesome details,” said Sue, “but these girls are very young and slight and their bodies are simply not designed for that.”

Her patients were deeply ashamed at presenting with such injuries. They had lied to their mums about it and felt they couldn’t confide in anyone else, which only added to their distress. When Sue questioned them further, they said they were humiliated by the experience, but they had simply not felt they could say no. Anal sex was standard among teenagers now, even though the girls knew that it hurt.

There was stunned silence among the mothers around that dinner table, although I think some of us may have let out involuntary cries of dismay and disbelief.

For Sue’s surgery isn’t in some inner-city borough where kids may have been brutalised or come from cultures where such practices are commonly used as contraception. Sue works in the leafy heart of Hampshire. The girls presenting with incontinence were often under the age of consent and from loving, stable homes. Just the sort of kids who, only two generations ago, would have been enjoying riding and ballet lessons, and still looking forward to their first kiss, not being coerced into violent sex by some kid who picked up his ideas about physical intimacy from a dogging video on his mobile.

The harm, of course, is not just physical. A study this week revealed that the number of schoolgirls at risk of emotional problems has risen sharply. Scientists for the Journal of Adolescent Health were surprised to see a 7 per cent spike in only five years among girls aged 11 to 13 reporting emotional issues. Boys remained fairly stable while girls faced “unique pressures”. Researchers said the causes could include the drive to achieve an unrealistic body shape, perpetuated by social media and an increasing sexualisation of young women.

Girls have always starved themselves in order to be more lovable, or maybe to have less of themselves to hate. The dreadful, recent death of Eloise Parry, a beautiful 21-year-old student poisoned by illegal “diet pills” in an attempt to keep her super-slim figure, is more proof than we can bear of that.

What is new and dangerous is the ability to post selfies, then wait for approval to come flooding in. You don’t have to spend long with an insecure teenage girl (is there any other kind?) to work out that her happiness is tremulously yoked to the getting of Likes or little lovehearts on Facebook or Instagram.

Take that female insecurity, warp and magnify it in the internet Hall of Mirrors, add a longing to be “fit” and popular, then stir into an ubiquitous porn culture and you have a hellish recipe for sad, abused girls.

It explains why more than four in 10 girls between the ages of 13 and 17 in England say they have been coerced into sex acts, according to one of the largest European polls on teenage sexual experience. Recent research by the Universities of Bristol and Central Lancashire found that a fifth of girls had suffered violence or intimidation from their teenage boyfriends, a high proportion of whom regularly viewed pornography, with one in five boys harbouring “extremely negative attitides towards women”.

• Why it takes 30 minutes a day to tame a teenager

The end result is what Sue sees in her work as a GP. Young girls – children, really – who abase themselves to pass for normal in a grim, pornified culture. Another study of British teenagers found that most youngsters’ first experience of anal sex occurred within a relationship, but it was “rarely under circumstances of mutual exploration of sexual pleasure”. Instead, it was boys who pushed the girls to try it, with boys reporting that they felt “expected” to take that role.

Moreover, both genders expected males to find pleasure in the act whereas females were mostly expected to “endure the negative aspects such as pain or a damaged reputation”.

You don’t need to be of the Mary Whitehouse persuasion to feel that something has gone catastrophically awry here. I’m still recovering from a tutor at my daughter’s sixth-form college telling me he thought that at least a third of the girls in her year were depressed or self-harming. How much more proof do we need that young women are in crisis?

Claire Lilley, the head of child safety online at the NSPCC, says the latest research should be a wake-up call to government to make sure teenagers receive clear teaching about healthy sexual relationships. You bet it should. Demonstrating how to put a condom on a banana just won’t cut it anymore, not when tens of thousands of girls are revealing “serious distress and harm following abusive behaviour from boyfriends”.

• Binge-drinking teenagers are 'damaging their brain development'

Mature women can generally make up their own minds about what they are and aren’t prepared to do in bed. That is a private matter among consenting adults, although I don’t know a single woman who thinks that a man insisting on anal sex is anything other than a depersonalising act of aggression. For inexperienced teenage girls it’s a different matter. Their whole sexting culture sends them one crude, insidious message: buggers must be choosers.

However embarrassing it may be, we need to educate and embolden our daughters to fight back against pornography, which is warping the behaviour of boys who are supposed to be their lovers, not their abusers. Anything that hurts and humiliates you is never OK. I suggest that future sex education classes begin with this joke:

“I asked my wife to try anal sex. ‘Sure,’ she said: ‘You first.’”

PS: I just texted my own teenager for her view. She texted back: “A lot of truth to this. I think dubious consent is the greatest problem of my generation.”

ncG1vNJzZmivp6x7tbHLnp6rmaCde6S7ja6iaK%2BforKve8yoq6GdomLBsLrGrpxoaWFqgnWBmG5miaeio7yovsCpn7JlmJbAbq%2FHmqWgnZRiwamxjKWYp5yjmK6xsYyonWaZlKS5pr%2FCnqWcnV2Xsrq7zZ1kmqScYr%2Bmr86gpaKsmaS7b7TTpqM%3D