There is an extraordinary advert on television at the moment. It is for a range of hair colourants by L'Oreal, and every time I see it I feel a fresh wave of astonished indignation.

I don't usually think of myself as a fuddy-duddy. When it comes to duddies, I flatter myself that there are few as unfuddy as me. All the same, this is an advert so wantonly, pointlessly, cynically distasteful that whenever it lurches onto my screen I feel like putting on a cardigan and writing outraged letters to the newspaper, signing myself "Disgusted of Tunbridge Wells". Except of course I don't live in Tunbridge Wells. But you know what I mean.

As I understand it, the idea behind the advert, if calling it an idea is not unfairly insulting ideas everywhere, is that with L'Oreal you can change your hair colour. Not the most breathtaking idea, but what is breathtaking is the casual unpleasantness with which the idea is dramatised. A blonde woman strikes a variety of suggestive attitudes, trying like jiminy to get her eyes to twinkle in a playfully seductive fashion, while saying: "Faithful? Of course I am. But if something better comes along, I change." That is hardly even a double entendre. That is, at most, a one-and-half-times entendre.

Just as I am trying to persuade myself that this isn't really a high-gloss, large-budget attempt to elevate personal inconstancy to a positive virtue, the advert ends with its unequivocal injunction: "Be unfaithful. You're worth it."

'Be unfaithful. You're worth it.'

Surely you don't have to have been cheated on to be appalled by the empty hearts and cold eyes that sat down and dreamed up that sales pitch. It isn't just trivialising the idea of infidelity; it isn't just glamourising it. It is actually promoting it to a kind of self-actualising duty. It is like one of Oprah's self-esteem shows gone mad. Or madder. "Remember your spirit," urges Oprah. "Be unfaithful," urges L'Oreal.

I don't know why that ad offends me so much. Unfaithfulness offends me, but that ad offends me more. It is one thing to do something bad, but you should have the grace to acknowledge its badness. The Mistress (Discovery Channel; DSTV; Thursday; 1.30pm) was full of people doing bad things and pretending that badness was an old-fashioned idea whose time had passed.

The Mistress was a BBC documentary looking at famous cases of infidelity. It was not so much concerned with the kind of sneaking infidelity that has been around as long as monogamy, but with that brand of infidelity that proudly poses as honesty. In other words, that brand of infidelity that prefers the big lie to the string of little ones.

Percy Shelley was just such a liar. Never trust a Romantic poet, I always say. No one who spends that much time with skylarks and daffodils and nightingales should be relied upon by human beings. The show took us through Shelley's courtship of Mary Godwin. Mary was 18, and Shelley was married to his first wife Harriet. Besides the rather creepy detail that Shelley first declared his love to Mary beside her mother's grave (and, it is conjectured, immediately impregnated her upon the grave itself), Shelley distinguished himself by running that by-now familiar line that marriage and monogamy are outdated, stultifying tricks of bourgeois convention, and therefore the truly free souls, the truly modern souls, have a duty to themselves to reject them and follow the profligate whisperings of their passions.

Shelley, you will have gathered, was a little twerp. When he ran away to Geneva with Mary Godwin, his wife Harriet drowned herself, at which point Shelley the unconventional modern free spirit immediately took Mary as his wife, declaring to be faithful and true. Within a year or so, Shelley was of course doing to Mary as he had done to Harriet.

Begs the question of who Adam or Eve found to cheat with

Mary, who had composed Frankenstein while waiting to be married, never successfully wrote again.

Ted Hughes is another poet I wouldn't trust. Where the Romantics warbled about robin red-breasts and such, Ted Hughes brooded on crows and hawks. It didn't make him any better as a husband. In his avante-garde poetic way, Hughes took a lover. His wife, Sylvia Plath, left him, then gassed herself. Hughes married his lover in a blaze of conviction. A few years later, while he was out with yet another new lover, his second wife gassed herself and their four-year-old son.

There will always be infidelity. Catherine Deneuve appeared on the show to inform us that "Cheating is as old as Adam and Eve", which rather begs the question of who Adam or Eve found to cheat with. She's right, of course, especially today. But having a history does not ennoble infidelity. Unfaithfulness is a tatty little vice.

Admittedly very few loves last forever. Very few loves last past next Tuesday, if you really want the truth. There is no shame in the end of love. But the end of love is not a reason for infidelity. The end of love is a reason to end the relationship.

I know, rationally, that sexual inconstancy has very little meaning in and of itself. An extracurricular conjunction of the genitals has no necessary bearing on the primary love, or that quality of the marriage.

Monogamy, I would even suggest, is not natural to humans and especially to men. I don't care. Monogamy is one of the fine and beautiful gestures that we can make, both to our loves and to ourselves. It may be pointless, unnecessary and old-fashioned, but it is the more beautiful for that.

It is a gesture of faith, one of the last forms of idealism remaining to us. It is simple and silly and in most cases doomed to defeat, but to at least try is one of the ways that humans raise themselves above the animals. Or that's what I think, anyway. Mind you, I have never bought a bottle of L'Oreal. I guess I'm just not worth it.