All human actions have consequences, as we have seen amply illustrated in the world of money and economics. Bankers took risks that they thought they would never have to pay for; personal debts reached dismaying levels because borrowing and credit cards seemed easy. But consequences there will always be. And, as in the money markets, so in the field of personal relationships.

The spiralling rate of unwanted pregnancy – there are now 200,000 abortions annually, the highest number since legalisation in 1967 – as well as the exceptionally high rate of births to unsupported and very young mothers has prompted ever more energetic methods of addressing this problem. Schoolgirls in Oxford may now text the school nurse for morning-after contraception. Indeed, all women will soon be able to purchase this post-coital remedy in bulk over the internet.

And round-the-clock adverts for both condoms and easy access to abortion services are, it seems, soon to be advertised on national television.

Since sex is advertised so widely, and in so many highly pressurised and commercial ways, and to such very young people, perhaps there is something to be said for also highlighting the point that there are consequences to human conduct. But the trouble with the dual approach of recommending both condoms and abortion is that the messages seem to contradict one another.

One says "be prudent, take precautions" while the other says, in effect, "but there's an easily accessed remedy if a conception does occur". This is not so much the behavioural incentives of carrot and stick as carrot and carrot.

Both Marie Stopes and Margaret Sanger believed that workable contraception would halt abortion and unwanted births. Their reasoning, in this, was estimable, but perhaps insufficient attention was paid to the psychology of human motivation, which Dr Johnson captured more accurately when he said: "What is done easily, will be done frequently."

We all know that unwanted pregnancies occur for a wide number of reasons, from heedlessness to drunkenness, from bad luck to bad relationships, and not infrequently from an unconscious desire to test fertility – or to test those on-off relationships. And indeed not every "unplanned" pregnancy turns out to be an unwanted one.

But the sex education lesson that should be taught is just like the money-relationship lesson that should be taught: there will be consequences. An unhappy pregnancy, whether terminated or brought to term, is a consequence; and although individuals vary in their reactions to these events, sorrow and rue are not excluded. A sexually transmitted disease can, usually, now be cured, but it is still a consequence. Be aware of this truth.

Learn the old lesson of the money crisis: risks have consequences. It is mendacious not to make that point, in any advertising campaign affecting human choices and human interaction. It is also ineffective – as the teen pregnancy spiral has shown – to ignore deterrence while advancing incentives.