Perhaps I should start by declaring a possible connection: Ms Harman was at St Paul's Girls School at

Under the circumstances I shall make no further comment on this matter.
I remember the Paedophile Information Exchange, though. They had a nice sounding chap as their spokesman who said that some people wanted relations with adults of the opposite sex, some with people of the same sex, some with all kinds of kinky stuff, whilst they simply wanted to be allowed sex with a consenting child. One's first thought was that this seemed perfectly reasonable. It certainly sounded reasonable. It was only one's second thought that was 'Wait a minute! This is not normal, this is a disgusting breach of trust with a minor whom we deem unable to give his or her consent.'
So I can quite understand the assorted loony lefties in the National Council for Civil Liberties (it's just called 'Liberty' these days) thinking 'Well, we believe in fighting against persecution on the grounds of sexual preference, so obviously we should support these people who are being banned by the Government.'
In other words they couldn't see beyond their narrow world view: they put the Liberty part before the Civil part. P.I.E. became affiliated to the NCCL and Ms Harman helped it campaign for lower sentences for people in possession of child pornography. The NCCL's evidence to Parliament in 1976 said that a sexual experience with an adult caused no lasting damage to a child.
Ms Harman ought to go a fair bit further with her explanation of all this, but in the end she just needs to say 'I was a barking mad leftie at the time but hope I am more sensible now.'
But I fear the public is unforgiving and perhaps she should consider spending more time with her family.