I'm imagining I'm reading this at the time it was written in 1976 & nothing in it would have suggested legalising sex with young kids, I would have seen it at the time in the context of lowering the age of consent for gay sex to that of hetro sex, ie equality. At the time many, perhaps most parents would have been horrified if their teenaged son or indeed son of any age over that had been having a sexual relationship with another man. they would have regarded gays as 'perverts' & 'queers', these were terms bandied about by ordinary people chatting in pubs & at work etc at the time along with 'sending them fuckin Pakis home', it really was like this in provincial towns like Colchester. The Colchester pub, The Headgate with gay landlords & where gay men met in the front bar was known as 'the queers pub'. A local public toilet was regularly staked out by cops to catch cottagers, local residents campaigned for the demolition of these toilets & finally got their way. Popular TV programmes regularly took the piss out of gays. I'm trying to paint a picture here of the endemic anti gay feeling that existed at the time among ordinary working class folk.
Tbf although PIE was in the news, it wasn't like now, post Savile, stuff about paedos every day on the news, by now even 9yr olds must know what a paedophile is & if they don't they can use google. In those days to find out information about child sexual abuse would have required an afternoon at the reference library & I doubt there would have been many books about it. The daily & particularly Sunday tabloid press at the time were all about sexual titillation, 16yr old page 3 girls & which famous folk were shagging each other, sometimes with long lense photos, undercover reporters 'making an excuse & leaving' etc. The News of the World had always been known as 'the news of the screws' & articles about PIE were in the usual context of sexual scandal with the paper taking its usual hypocritical moral high ground stance. The main spokesman for PIE was Tom O'Carroll, iirc he appeared all about wanting to shag teenage boys & this is what PIE seemed mostly about. Obviously there were PIE members who wanted to sexually abuse young kids but my guess is Harman & co were zeroed in on the inequality of the age of consent for gay sex being 21 & the belief it should be lowered & saw this as an entirely separate issue to the sexual abuse of young children.