Lily Allen's open marriage woes confirm what I've known for a long time

Lily Allen does not come across as a particularly sympathetic person: somewhat entitled, a bit self-obsessed. But you have to feel a liitle for her: her marriage to David Harbour has broken down, on the grounds that he was unfaithful. But what have we here? It was an open marriage.

Of course it was all going to end in tears. Ladies, a word. Open marriages do not work. Nor does swinging, which some people these days seem to equate with something as innocuous as a round of golf. A generation of women in the 1960s was sold a dud steer and it's happening again.

Put bluntly, if a man wants to have sex with you, you have every right to demand exclusivity and deny him access to your body if he doesn't agree.

In the 1960s it was the sexual revolution - which none other that Martin Amis pointed out worked a lot better for men than it did for women - and these days it's the ghastly pornification of society which is leading young women to believe horrible sexual practices are normal when they are not.

Stand up and say so, though, and you risk being called a prude or worse. But it's not prudish to point out that for the majority of women sex involves emotion and that if your husband or partner betrays you that it's going to make you miserable. It is not prudish to expect and demand fidelity.

Instead we've accepted terms such as "hall pass" into the language and made it seem like the norm. I am not naïve: of course people cheat and I've known a few married women who were as bad as any man.

But it is a fact even in these modern and enlightened times, if a woman is going to bear children, she needs the father (not always the biological one) to stick around and make sure she's all right.

Various systems have grown up around the world to ensure this happens, primarily through marriage. And in ideal terms that means forsaking all others.

And I know that is not always the case. But it should be. Over 10 years ago a very sophisticated French art dealer told me it was preposterous that any man should confine himself to one woman after matrimony.

Guess what? His English wife, quite the racy lady, didn't agree. They divorced. Someone else seems to have taken a similar attitude: the Andrew formerly known as Prince. And look where that's got him.