If you ever talk properly with someone about where their relationship went wrong it's like joining them on a train journey to hell. They start off the same, chugging off innocently like some scene from The Railway Children, but then it's all 'Alight here for disappointment', 'We are fast approaching disillusionment', interminable delays and eventual derailments on the road to commitment. At this point, all most people have to show for their agony is a timetable showing where they thought they were going and where they actually ended up. Then the cry goes up: What went wrong? How did it become all about dirty socks and duty calls from the pub? Where did the romance go?

That's the reality of romance - it's as gaudy and perishable as a bunch of petrol-station carnations, and yet still we wail and grieve over every fading petal. Older, wiser, phooey: where romance is concerned, it seems that women in particular never get used to the sudden terrible absence of it all. You certainly never heard any woman say: 'Thank you for avoiding the cliché of remembering my birthday.' It's said to be part of the human condition to want what you can't have, but I would say that for women an even more powerful compulsion is to want back what they once had. With the latter, at least you don't get memories stinging you like angry bees when the romance starts to fade.

One problem is that men and women have wildly differing approaches to romance. In general, I truly believe that men are the more romantic, more open-hearted sex. When they like you, they really do just like you; there's none of that feverish totting up of pros and cons that rattle endlessly through the average pragmatic female mind from the moment they agree to share spit. There was even a sweet little report recently about how men were most likely to name their first kiss as their most treasured memory, while the women who were asked cited their first pet (as in Fido or Tibbles, not, as one man I spoke to gleefully thought, their first heavy petting session). Clearly men have a romantic streak a mile wide - the trouble is that this particular mile doesn't seem to go anywhere.

The problems begin because, like every horse I ever betted on in the Grand National, men only ever seem to start well. It's like once they've got you, once you're actually in a relationship, they truly believe that their work is done and they can put their feet up and make that relieved 'Ahh' noise. I have even been informed quite solemnly that a woman should beware the 'smarmy so-and-sos' who compulsively buy flowers for the ladies, as this is a sure sign they are closet wife beaters. Nice try, guys, but maybe women should be more wary of the man who never buys blooms, for he is a lazy thoughtless git. Or maybe it's men who should beware, considering the fact that another recent report states that more married women than ever are being unfaithful. A sign of the times probably - sexual opportunism will always flourish where there is greater sexual opportunity and women are more mobile and independent than ever before - or maybe this story is older than that.

It seems to me that, like many men before them, women are starting to choose the bits of marriage they like (stability, companionship) while opting to ignore or dodge the more tedious bits (fidelity, compromise). In the past, your average woman would probably have spent long periods of her marriage haunted by the great howling ghost of 'Before'. As in: 'He would have given me that lift before', 'He wouldn't have let me carry that heavy bag before', 'He would have remembered our anniversary before.' All those horrible jarring moments usually dismissed by men but which loom large in a woman's life, chipping away at her sense of romantic justice. All very 'You don't bring me flowers any more', but heartbreaking nonetheless.

Now it seems that there's a new generation of women coming through who refuse to be haunted by 'Before,' preferring to concentrate on 'What and who next?' As it is mainly women who decide to end relationships, it makes a terrible kind of sense that they are the gender to redefine the marital boundaries once more. The message is clear: If romance is what these women want, then romance they'll get, and men really will have to try to keep up. After all, Diana was wrong.There are never so few as three people in a marriage. There are at least four. The people you once were, and the people you became after the romance died.

· barbara.ellen@observer.co.uk