ONE OF THE basic tenets of British law is that ignorance is no defence. It’s no use saying to a policeman: “I’m sorry, officer. I had no idea I wasn’t allowed to drive at 50mph down that residential street.” He will book you regardless.

So why is the Lord Chancellor now proposing that the law should be changed because of people’s ignorance?

Lord Falconer of Thoroton has asked the Law Commission to draw up legislation giving cohabitees the same, or similar, rights as married couples. He and his supporters draw on sob stories of cohabiting partners who find, to their dismay, that after their partner dies or leaves them, they are much worse off than if they had been married.

According to researchers at the University of Bradford, 59 per cent of cohabitees believe that if they live together for a period of time, they have a common-law marriage which gives them the same rights as married couples. They are then horrified to discover that this is not the case.

Cohabitees cannot inherit their partner’s assets free of inheritance tax. They don’t always have parental rights over children. They are not automatically entitled to a share of finances after a split or to their partner’s pension after he or she dies. And they don’t count as legal “next of kin”.

But why should we change the law to allow them these rights? If cohabitees want to be treated like married couples, there is an easy answer: let them get married. And if they are determined to remain unwed, then why not just educate them in the legal costs of doing so?

Civil partnerships for committed gay couples are a great idea. If they plan to devote themselves to each other for life, then they should be allowed the same legal protection as married couples. But heterosexual cohabitees already have the option of marriage. They don’t need more rights; they just need better education so that they are not deluded into believing that the law recognises their relationship.

If we were to allow cohabitees all the privileges of marriage without any of the costs, such as messy and expensive divorces, then many fewer people would get married. In fact, there would be a positive disincentive to getting hitched. And that would surely be a bad thing, particularly for any children involved.

All the evidence shows that cohabitation is less stable than marriage. A study for Civitas, the think-tank, found that 75 per cent of married relationships last for ten years or more, compared with just 18 per cent of cohabiting ones.

Of course, there is a large element of self-selection here. It is those couples with the greatest commitment who choose to marry in the first place. Nonetheless, there is a qualitative difference between how you feel when you are cohabiting and how you feel once you are married.

I know. I have tried both. And in my experience, cohabitation almost always has a contingent quality to it. You think to yourself: “I’ll stay in this relationship as long it makes me happy/as long as I’m still in love/as long as things are going well.” You know that you can always leave if you feel like it.

As a result, your time horizons are much shorter. When I was cohabiting, I never thought about growing old with my boyfriend — it was all I could do to think beyond the summer.

Marriage feels quite different. Yes, of course, divorce is available, but it is an absolutely last resort. You have promised to stay together for better, for worse; for richer, for poorer; in sickness and in health. If I were unhappy in my marriage, I wouldn’t just leave; I would try to do something about it within the marriage.

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