Such a charming little story I heard this week, told me by a friend who is governor of her local school. We were standing outside Sainsbury's, wondering where the car was parked, when The Twelve Days of Christmas started up in the forecourt. My friend sighed; gave her head a sad little shake. "What?" I asked. Well, she said, as a treat for the younger pupils, two officials from the local NHS trust had arrived to sing a song for them during the class that used to be sex education and is now PSHE (personal, social and health education). It was a jolly song, chosen with the festive season in mind, and because I've since found the lyrics online I can tell you that it went like this:

"On the first day of Christmas, my true love gave to me, a bug that made it hard to pee...
On the second day of Christmas my true love gave to me chlamydia and a chance of infertility...
On the third day of Christmas my true love gave to me, syphilis and now my testicles are sore and lumpeee..."

On days four to 12, sang the happy health workers, my true love gave to me, in this order: genital herpes, gonorrhoea, pubic lice, hepatitis, trichomoniasis, genital warts, scabies, pelvic inflammatory disease and crabs. The song is called The 12 STDs of Xmas and I think (though I hope I'm wrong) it was accompanied by slides.

Afterwards, the NHS minstrels left very pleased with themselves – instructive and fun too! But the kids sat in worried silence. They were traumatised, said my friend. No wonder. Their spongey little brains were learning to associate true love with herpes and Christmas with lumpy testicles.

But if the 12 STDs of Christmas didn't enlighten the pupils, it has at least helped me to pinpoint the source of my unease about this sort of "warts-and-all" sex ed, which nice Mr Balls has announced this week will now be mandatory even for five-year-olds.

I don't think the STD song was an aberration – it's the sort of thing sex ed enthusiasts think is tops. And I don't think they really mind if it horrifies children, because the kids are beside the point.

Aggressive PSHE, of the sort pushed by Labour for a decade, isn't really intended to help children – how could it be? Where's the need for sex ed in the 21st-century Britain? There's sex on movie screens, on billboards, in magazines; sex in pop songs trickling down iPod wires straight into auditory cortices of every sentient tot.

Where my generation once pored like medieval scholars over the pages of Judy Blume's Forever looking for clues, a 21st-century teen (from any background) just asks Google, the purveyor of hardcore, no questions asked. Of course, PSHE hopes to teach not just the facts of life but a responsible attitude towards them – but it's wasting its time there too. Since when did telling off a child to be responsible have any effect?

You can teach a teen to take things seriously by putting it in charge of something, or leaving it on open moorland and making it find its way home. But lectures just wash over them.

The stats say it best: the Government first began its sex ed crusade in 1999 but to date it has had zero effect on teenage conception rates. For a decade, the Government has scatter-bombed British kids with condoms. Buckets-full in schools for all ages (a choice of coloured, flavoured and ribbed, says my friend); lessons in how to put them on ("Darling, what a wonderful technique, your father and I are so proud!"). In 2002 during some new initiative, there were condom-dispensing NHS vans kerb-crawling for kids.

None of it has had the slightest effect. STDs are on a soaring all-time high, as are teenage abortions. A reputable study done in Scotland proved that girls who underwent a prolonged and enhanced sex ed programme of the sort Balls has just prescribed had no better pregnancy and abortion rates than the girls who didn't.

I wouldn't be surprised if it even proved to be damaging. My governor friend told me of one boy who was too embarrassed to roll a rubber on to a banana in front of the girls and kept crying. Could they have a sex ed class just for boys? asked his mum. No way, said the PSHE teacher. That would be wrong. But why would it be wrong? What's so bad about avoiding humiliation?

And why all this prescriptive bossiness over procreation? I've come up with two answers. The kinder answer is that government ministers and leftie teachers are still fighting ghosts of the past. The sex ed pests come from a generation that was still suffering from the after-effects of the Victorian sex omertà. They had to rage against prudery and prejudice in the 1970s and have been fighting ever since. For them, each condom in a seven-year-old hand is one in the eye for their frosty parents.

The second reason is more cynical. It remains stubbornly true that it's girls and boys from poorer backgrounds, early school-leavers and benefit-seekers who have all these unwanted pregnancies and STDs. But improving schools, raising aspirations and lowering unemployment is a daunting and difficult job. Much easier and much cheaper too just to carry on throwing condoms around.

Bercow's bijou riverside home

There are many good reasons to be wary of young politicians, but I hadn't realised until recently that to top it all, they stand to cost us a good deal more than their wise old colleagues. John Bercow, the Tory Speaker, has the best flat in London – a glorious two-storey apartment in the House of Commons overlooking the Thames.

I was looking at it enviously the other day when a Commons caretaker told me that it was going to cost an extra 20 grand of public cash to make it child-friendly. Then there's the thought that, if he makes it to PM, Cameron stands to be the youngest Prime Minister since 1812. But a young PM means young kids and, as Blair found out, it's almost impossible to fit children into No 10.

Unlike Blair, however, Cameron can't just take over No 11 because Chancellor George has two children under 10 as well, and there's talk of a third to come. I sense more expensive extensions to already flash pads, and my bitter, taxpayer's heart brims with resentment.

Why all the fuss to install Park on the plinth?

So Air Chief Marshal Sir Keith Park has made it up on to the fourth plinth in Trafalgar Square at long last. Arise, Sir Keith, good job. I salute you as I cycle past, for without you (I'm told) the Battle of Britain would have been lost and the free world a goner.

OK, I'm lying. The truth is, I'd never heard of Keith Park GCB, KBE, MC before the campaign to plinth him began, and I still can't quite figure out what all the fuss is about.

The campaign was frighteningly well organised and well funded: field marshals, MPs, Tony Benn, the vice-chancellor of Oxford – they've been at it for years, pushing for Park, but why?

Surely there are other, better known and just as heroic or deserving candidates. What about the Queen? There are other questions too. What were the Parkies thinking when they chose to depict their hero pulling on what looks like a pre-op surgical glove? And why such intensity and lack of humour?

When Boris Johnson was first elected Mayor of London, he had to sit through interminable and vicious debates between the various fourth-plinth competitors. During one such, he was struck with inspiration. "I've got it!" he said. "Let's forget the silly old statue and just rename Hyde Park as Keith Park!"

It's an excellent joke I think, not to mention a terrific idea. Not one Parkie even cracked a smile.

A sad time when Leonardo sells his soul

Why do beautiful, rich, talented actors – especially cool ones who make a show of their artistic integrity – go and ruin it by appearing in watch adverts? A crack appeared in my teenage heart when Johnny Depp first posed for Mont Blanc pens. Now there's lovely Leo DiCaprio, looking all dangerous and moody in the service of Tag Heuer. It's perplexing and wrong. Surely it's the rest of us (skint and unprincipled) who have to kowtow to big business. Isn't the whole point of being glam and rich that you can afford not to sell your soul?