Even the word is wrong. The literal meaning of "paedophile" should be someone who loves children, but few would say it is love that drives such a person. A paedophile does not love children; he abuses them. Still, paedophile is the word we are stuck with, this week especially. Exhumed from the 1970s has been the Paedophile Information Exchange and its unexpected link to the National Council for Civil Liberties, a disinterment that has discomforted senior figures in the Labour party and brought a reminder that when it comes to the past, especially of politicians, it is, as Faulkner said, never dead. It is not even past. Instead it can return decades later, with venom.

It's a reminder too that what can seem enlightened and progressive in one era can look very different years later. It's a shock now to read, for example, that the great luminaries of the pre-war intellectual elite, especially on the left, were in thrall to eugenics, a theory of human selection which strikes us today as morally indefensible but which, at the time, was nothing less than the common sense of the age.

Merely saying, "It was a different time" doesn't work as a defence, a point that Patricia Hewitt has wisely understood. She has apologised for the decision she took, as general secretary of the NCCL, to put her name to a March 1976 press release, proposing that the age of consent be lowered to 14 or even 10 "where consent of a child … can be proved". The NCCL document further argued that incest between consenting persons was no crime. Now Hewitt admits: "I got it wrong on PIE and I apologise for having done so."

Some will praise Hewitt for being smarter than Harriet Harman, the former legal officer of the NCCL whose responses to the PIE revelations have been more tortured. But this is not a matter of superior judgment. Hewitt surely had no option but to admit her error and show contrition. As the boss, and with her name on that document she was much more closely implicated than Harman has been. Proposing that a 10-year-old can consent to sex with an adult is far more serious than any action so far attributed to Labour's deputy leader. Still, it would have cost Harman nothing to say, the instant this story broke, that the NCCL's affiliation arrangements were clearly a mess if they allowed a group such as PIE to join. Perhaps the fear of conceding even an inch to the Mail enemy blinded her to that obvious point.

What neither Hewitt nor Harman can quite say is that the Mail's attack depends on an unspoken assumption that the NCCL crowd were uniquely indulgent of PIE and paedophilia – that, while everyone else back then shared the revulsion we feel today towards child abusers, these crackpot lefty libertarians were inexplicably permissive. But that assumption is wrong.

The clue is in the name. The activists of the PIE did not hide who they were: they put the word "paedophile" on the tin. They had named spokesmen and a letterhead (featuring a line-drawing of two bare-limbed children on a rock). The existence of such an unashamed group is unimaginable now. "Paedophile" is the worst possible insult, languishing in the moral hierarchy somewhere between "racist" and "murderer", if not, in fact, below both of them.

Yet it was clearly not that way 40 years ago. The PIE men felt they could hold their heads up, not just among the libertarians of the NCCL but in British society in general. PIE did interviews with the media. A colleague recalls seeing a PIE stall at a university conference. If you called the PIE members' hotline, it was answered by a chap who worked at the Home Office.

It's hard for us to credit it, but it seems paedophilia did not carry quite the radioactive stigma it does today. Or maybe it's not so hard to understand. For what was one of the recurring themes of the Jimmy Savile and Stuart Hall revelations? That these men hid in plain sight, that their excessive interest in very young people was barely concealed: in Savile's case, he all but molested one girl on camera.

By their actions, these men signalled their confidence that society was ready to look the other way, if not to indulge their behaviour. That they got away with their crimes for so long suggests they were not wrong.

This is perilously awkward territory, but a look back at the popular culture of earlier eras reveals the difference with today. When Shirley Temple died earlier this month, several critics noted that her best-known performances – say, the toddler singing The Good Ship Lollipop before a watching group of adult men – now look downright creepy. Some noticed it at the time. In 1938 Graham Greene was sued for libel after writing a review of one Temple movie that argued that the child was presented as a sex object: "Her admirers – middle-aged men and clergymen – respond to her dubious coquetry, to the sight of her well-shaped and desirable little body, packed with enormous vitality, only because the safety curtain of story and dialogue drops between their intelligence and their desire." When Hewitt and Harman were at the helm of the NCCL, one of the hit songs of the age was Gilbert O'Sullivan's Clair. Doubtless, it was no more than an innocent tribute to a friend's young daughter but no singer would dare write such lyrics today: "You're more than a child, oh, Clair … But why in spite of our age difference do I cry each time I leave you / I feel I could die / Nothing means more to me / Than hearing you say / I'm going to marry you."

The point is, it's more than the attitudes of the NCCL that strike us now as alien: it's the attitudes of that whole era. Our shock is infused with a kind of generational smugness: look how much we've evolved since then. Some of that is richly justified. It's a step forward that we now understand what clearly eluded many liberals at the time: that there can be no such thing as meaningful "consent" from a child when it comes to a sexual encounter with an adult. It is abuse.

But there should be a limit to our generation's self-satisfaction. We have our own confusions. We believe a child cannot consent to sex until 16; yet we say they are old enough to be criminally responsible at 10. Nor is today's pop culture free of the habit of seeing children sexually: witness, as Harman has rightly pointed out, the Mail's drooling coverage of celebrity kids and their bodies.

Above all, while we may be better at shunning paedophiles, we're not so good at convicting them. We may have succeeded in driving child abusers underground, but the problem is still there – manifested in the proliferation of child porn on the internet, in underage sex-tourism and, tragically, in thousands of families every day of the week.

The glum truth is that this is a sickness that clings to our society. It did then and it does now. Exposing the blind spots of would-be politicians 40 years ago allows us to pretend that this ailment somehow belongs in our past. It doesn't. It lives with us still.

Twitter: @Freedland