When the Internet Watch Foundation blocks access to a website containing a �potentially illegal indecent image of a child�, there�s usually nothing more to say than �well done�.

When the Internet Watch Foundation blocks access to a website containing a "potentially illegal indecent image of a child", there's usually nothing more to say than "well done". But when the picture in question has been widely viewed and perfectly legal for 22 years, it's worth stopping to ask what has changed, and why?

The image blocked over the weekend was the cover of a 1976 album, Virgin Killer, by the German heavy metal band The Scorpions, featuring a naked, pubescent girl. It created controversy at the time, and was replaced in several territories, including the US. But it was never banned and is still being sold as part of a CD boxed set. So why is it only now suspected of being illegal?

There is little doubt that we have become much more nervous about children and sex, to the extent that there seems to be automatic suspicion of any image of a naked minor. Last year, for example, Northumbria Police seized a photograph by Nan Goldin from the Baltic Centre for Contemporary Art in Gateshead. A few months earlier, Australian police removed photographs by Bill Nelson from the Roslyn Oxley9 gallery in Sydney. In both cases, the idea that the nudity of the very young children portrayed was in any way erotic was absurd.

The Scorpions cover, however, cannot credibly be defended on artistic or moral grounds. "Looking at that picture today makes me cringe," the band's former guitarist Uli Jon Roth said two years ago. "It was done in the worst possible taste."

He's right. But the fact that it is now considered not only tasteless but potentially illegal reflects how much more aware of and worried about child sex abuse we are than we used to be. For instance, a report published in The Lancet last week by researchers at University College London's Institute of Child Health suggested that between 5% and 10% of girls and 1% to 5% of boys are subjected to sexual abuse, usually by a family friend or relative.

Two things are striking about how this statistic has been reported and received. One is how ready most people were to believe what would have been unbelievable not so long ago. The second is how it lumps together cases ranging from abuse against infants to sex between minors and adults whom they describe as boyfriends and girlfriends, which is still wrong, but not in the same league of depravity.

These two facts reflect important features about moral climate change. One is that we acclimatise quickly, and soon forget how things didn't always feel this way. The Scorpions' album cover, like Life on Mars's Gene Hunt, reminds us that what outrages one generation may barely stir another.

More importantly, they show how moral crusades can suffer from "mission creep". When society gets especially preoccupied with anything - reds under the bed, racism or child abuse - it can start to see it everywhere. Shades of grey disappear and anything connected with the vice of the age is seen as equivalent to it.

This could be happening with child abuse. One of the reasons we find paedophilia so repulsive is that it destroys the innocence which is the right of any child. Ironically, however, when we are too afraid of paedophiles we become unable to see the very innocence we want to protect.

Although I'm glad that I can't imagine a record cover like Virgin Killer's being released today, I'm worried about the confused thinking about sex and children that would make us go so far as to criminalise the cover.

I don't want to turn back the clock to 1976, when the Race Relations Act was only just being passed. The lights which have come on gradually over history, exposing society's shames, must not be turned off. But we have to adjust our moral vision so that living in the dark is succeeded by something better than being blinded by the light.