Mature women can generally make up their own minds about what they are prepared to do in bed. For inexperienced teenage girls it's a different matter.

Sometimes you hear a story that is so awful, 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, look away now.

I was having dinner with a group of women when the conversation moved on to how we could raise happy, well-balanced sons and daughters who are capable of forming meaningful relationships when internet pornography has changed the landscape of adolescence beyond recognition.

A couple of the women said 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."

DEEPLY ASHAMED

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 they wanted to, or because they enjoyed it, but because a boy expected them 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 it hurt.

There was stunned silence around that table, although I think some of us may have let out involuntary cries of dismay and disbelief. Sue's surgery isn't in the brutalised inner-city but in a leafy suburb.

The girls presenting with incontinence were often under the age of consent and from loving, stable homes. Just the sort of kids who, 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.

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The harm is not just physical. According to a recent study, 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 to be more lovable, or maybe to have less of themselves to hate. What is new and dangerous is the ability to post selfies, then wait for approval to come flooding in.

'PORNIFIED CULTURE'

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 13 and 17 in England say they have been coerced into sex acts, according to one of the largest European polls on teenage experiences.

Research by the universities of Bristol and Central Lancashire concluded that a fifth of girls had suffered violence or intimidation from teenage boyfriends, a high proportion of whom regularly viewed pornography, with one in five harbouring "extremely negative attitudes towards women".

The end result is what Sue sees as a GP. Young girls – children, really – who abase themselves to pass for normal in a grim, pornified culture.

According to another study of British teenagers, 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".

EDUCATE AND EMBOLDEN

You don't need to be of the Mary Whitehouse conservative 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.

Mature women can generally make up their own minds about what they are 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.

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 texted my own teenager for her view. She texted back: "A lot of truth in this. I think dubious consent is the greatest problem of my generation."