Imagine, just for a moment, that the Government wanted to keep a record of everyone's sexuality. They need to know this detailed demographic data because it will be highly useful in civic planning. It will help them work out what provision needs to be made for sexual health services, how many children are likely to be born, how many schools to build, etc.
You trust the Government, you voted for them, you and your friends have nothing to hide with regards to your sexuality.
But! Shock horror! After creating the database, the Government loses the election and the homophobes at UKIP get in to power!
Now they have a database of every gay in the village, and can harass then, try to "cure" them, or make their lives a living hell.
Far fetched? Not really. With Cameron's inane web filtering plan, the "black boxes" in ISPs which can record every click you make, and the selling of the your NHS details to private parties, we're in a situation where a malicious government could cause serious damage to us.
The security expert Bruce Schneier wrote a wonderful article for CNN on how the existing surveillance state is leading to disastrous breaches of our private information. He concludes by saying:
It's bad civic hygiene to build technologies that could someday be used to facilitate a police state.
-- Bruce Schneier on CNN
We have to be careful that the apparatus we build cannot easily be misused for evil purposes. Sure, even an innocuous toaster can be weaponised if someone is willing enough, but we should not fall into the trap of making systems which can easily be turned against the people.
It's probably sensible to build a database of which car belongs to which owner - it has an important civil use and would be hard to abuse (although not impossible).
Should we have a national database of, say, religious beliefs? Almost instinctively the answer is no. The memories of fascist dictators haunt our collective consciousness. We have seen countless times how race and religious identity become death penalties. We wouldn't countenance it.
Civic hygiene isn't about saying we distrust our current government - it's about not trusting the next government.