Amusingly, several articles from around the time that Harman and Hewitt were being criticised five years ago (deserved in the case of Hewitt, less so in the case of Harman) add to the list of obvious political smears the then-current accusations of the German Greens having been taken over by paedophiles. The German Greens are now tearing themselves to pieces dealing with the aftermath of, you guessed it, being taken over by paedophiles.
Nicely brought up middle class radicals, who think that being a communist is a way to get a rise of their parents, assume that everyone they meet is equally nicely brought up. So organisations which are well intentioned are joined by the naive - NCCL, CND, Green Party, Scout and Guide movement, anti-vivisectionists, most political parties on the left - who cannot then see that there are others who are rather nastier who are using the group for their own purposes.
Hence such organisations are variously rife with paedophiles, anti-democratic Trots, violent anarchists, etc. The structures of the movements make entryism dangerously easy, as the majority of the members are predisposed to think well of people.
So, sadly, the more well-intentioned an organisation, the more likely that it contains very bad people intending to do very bad things.