I lived through the 70s and I agree with those who have said the cultural, social and sexual mores of the time were different. That is not the same as saying paedophilia, in the true sense of the word, was widely acceptable and I think the fact that when PIE's ideals WERE given the oxygen of publicity and exposed to some form of scrutiny, they were seen as abhorrent and subsequently shut down, confirms that to be the case.
It isn't helpful though to apply 2014 standards to 1970s attitudes and decisions. Today, a paedophile is THE bogeyman extraordinaire, the most demonised creature of our society (well, possibly along with an Islamic fundamentalist). Back then, the term and the concept, as we understand it now, wasn't in wide usage. You had your local Dirty Old Man and your pervert, but they were as much figures of fun / ridicule as they were of fear. In my experience, the understanding of what they might actually DO was vague, formless, unexplained. This was a generation who hadn't yet lived through the cumulative effects of the care home child abuse scandals, the Roy Whitings of the world (so far as I am aware, the term "paedophile" hadn't been applied to e.g. the Moors Murderers at this point, despite the obvious sexual element of those crimes - it just wasn't in that sort of general use), Operation Ore and the power of Internet pornography.
I would also say that ironically, while the word itself wasn't in widespread use, there was probably more of a distinction in those days between true paedophilia and fancying teenagers, whereas society nowadays tends to apply the term to anyone who has sexual activity with someone below the age of consent. Today, if a man sleeps with a girl aged 15 years and 364 days, someone will call him a paedo. Back then, there was more of a blind eye turned. A post-pubescent girl (and the average age of hitting puberty was higher than it is today) who dressed older, wore make-up and was sexually "aware" was judged to "know what she was doing". Mandy Smith would have passed for 18 in appearance when the Bill Wyman story broke, had apparently consented and so a lot of shoulders were shrugged. It was shocking, but I remember it being presented in certain media quarters as "titillatingly shocking" rather than "morally reprehensible" IYSWIM. There wasn't the same widespread understanding or agreement about the inability of children to consent - you could argue it was some of the uncomfortable questions raised by PIE's activities which partially informed our current thinking on this point - and so between that, and the power wielded by men, cases of post-pubertal but underage and nominally consensual sex were rarely, if ever prosecuted.
Although I wasn't personally involved in politics or civil rights activism at that time, I can understand through my knowledge of those times, how an organisation like the NCCL, given what it stood for, could - without the luxury of hindsight - come to think that PIE's agenda should at least be considered/discussed, even if the ideas were morally repugnant to the individuals. I can easily imagine that there were probably some very of-the-age conversations along the lines of "I don't agree with what you say, but I defend to the death your right to say it". That isn't the same as saying I agree with or condone what was done, but the abhorrent nature of paedophilia doesn't blind me to the historical context in which it was done.
I think Harriet Harman is being badly advised by party advisors who have misjudged the situation, who possibly have assumed that the antipathy towards the DM following the Milliband Sr debacle would carry them through this issue and who seem unable to appreciate that an acknowledgement of "I'm sorry, we did what we thought was right at the time but of course, looking back, we were very wrong" regardless of the timings of this document and that statement and the other submission yadda yadda yadda would be more greatly respected than her response to date.