She is “livid and shocked”. He has had a “real ear-bashing”. And now we all know that the husband of the Home Secretary Jacqui Smith appears to use pornography. The pay-for-view imbroglio besetting Smith and Richard Timney has unfolded in an excruciatingly public manner: Timney has now apologised to the nation for mistakenly claiming a £67 satellite television bill that included two pornographic films on Smith's parliamentary expenses. One anonymous “close friend of the couple” told a reporter that Timney “will be sleeping on the sofa for a while. To say she's angry with her husband would be an understatement”.

This story has two elements; pornography and expenses. It is the expenses that put Smith's political life in jeopardy. If you can, though, ignore for a moment the self-evident wrongness of a regime that permits MPs to charge you and I for their kitchen sinks (Smith's is stone, £550), “entertainment centres” (Smith has two widescreen TVs, with two Digiboxes, £1,100) and the pornography watched on those entertainment centres (even if charging for it was, in retrospect, erroneous).

It is the public revelation about her husband's use of pornpgraphy that will have stung the Home Secretary at the most personal level. As the relationship expert and Times columnist Dr Pam Spurr says: “It is extremely grim for someone to discover their dirty laundry within their own relationship, let alone to discover it at the same time as the whole country.”

Across the nation, though, many will discreetly acknowledge an affinity to Smith and Timney: in a survey last year 74 per cent of relationship counsellors said that it was increasingly common to see excessive use of internet pornography as a problem in relationships.

Susan Quillam, a relationship psychologist, says that her experience chimes with that of her peers. When a husband is using pornography and his wife is offended by that, she says, it can be symptomatic of profound problems within the relationship: “One could be that you may have different sexual preferences, one of you may be gay, for instance, and the other not. Or there may be disagreement on whether or not you want children. The third one is if you have serious, intractable disagreements about what is sexually acceptable.”

For husbands caught secretly consuming pornography, there is, says Quillam, one effective way of rescuing the situation: “For women the issue is often ‘he was watching porn - am I not pleasing him?' If he can genuinely make the distinction between ‘I like to bring myself off by looking at pictures, but I love you and we are still having sex - it is mindless and you are the one I love', then she may say ‘OK, fine'.” Should the couple have stopped having sex, however, the problem can be more intractable unless he stops watching it - and sticks to that commitment. If he does not, and is discovered, says Quillam, “that is often the marriage-breaker”.

One anonymous married user of pornography said that his habit, as yet undiscovered by his wife of ten years, was something originally conceived as a “favour” to her: “We had been arguing a lot and rarely having sex, and I was increasingly tempted to sleep with someone else. Porn is a way of getting that satisfaction without physically betraying your wife.”

Yet according to Quillam, some women find that discovering their husband's porn habit is more disturbing than if he had a mistress: “I get letters that say: ‘If he had had an affair I could understand it, because that would have been with a real woman and he could have controlled it. But all these pneumatic blondes who are drop-dead gorgeous and digitally altered up to the eyelids: it is an endless stream, he can't say to me now it's over because it will never be over'.”

In Smith and Timney's case, it was pornographic TV programming delivered by satellite. This, however, is a relatively conservative way of viewing; most men who like porn use the internet. Alexa.com charts the most popular websites on a nation-by-nation basis. Yesterday it ranked pornhub.com the 23rd most popular site in Britain, with redtube.com (“portal gigante de videos de sex”) ranking 30th. YouPorn.com was the 32nd most popular. To put that in perspective, the National Lottery is ranked 52nd, Tesco 61st and the Jobcentre 67th.

Yet, of the married men using those sites, at least some of wives will be aware and tolerate the pixelated third-parties in their relationship, says Quillam. She adds: “I know of at least one case where a husband went to a wife and said: ‘We are no longer having sex. I love you very dearly. I want to honour your choice not to have sex. Rather than have an affair I want to tell you that I am going to use internet porn.” She cried a lot and she said to me ‘I'm glad he was honest with me and in the end I had to say thank you, this is a real sign of love'.”

Not all pornography is watched in secret - some wives participate. Certainly, the type of pornography watched chez Smith and Timney, broadcast as it was on the television, will have been on the lighter side of fruity. And, ultimately, who apart from Jacqui Smith and her family, can know for sure who ordered and watched those films? It is on the internet, where more British fingers are clicking on YouPorn than Tesco, that the really damaging subject matter is to be found.

Staff at the Tavistock and Portman NHS foundation trust in London have been treating a growing number of men referred after watching illegal forms of pornography, including paedophilia. According to Stanley Ruszczynski, the clinical director: “Even with soft, normal, sexual pornography there is the issue about addiction.The whole thing about relationships is that you have to deal with the reality of the person - you have to negotiate, compromise or adapt to someone's needs.

“With a piece of pornography you can switch it on, switch it off, move it forwards and backwards, and choose whatever nature of person you want to see. There is a fantasy of omnipotence, a fantasy of being in charge. And that is very anti-relationship, isn't it? No real human relationship is like that.”

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