Remember the good old days of Tory sleaze? In the dying days of John Major's Back-to-Basics era, Mr Merchant was caught on tabloid camera kissing and fondling a young woman in a park. He managed to persuade his wife, and, more importantly, his constituency party, that things were not as they looked, but six months later the Sunday Mirror obtained pictures of the pair in bed. Mr Merchant was forced to resign as an MP.
If Mrs Merchant is perplexed in her attempt to pin down the complex formula which governs the political consequences of sexual betrayal, she is not alone. The events of the coming few days will add a little more case law to the puzzling precedents in the field.
Will the Foreign Secretary finally get his comeuppance for dumping his wife at Heathrow on the eve of their annual holiday? Or will the Prime Minister's dismissal yesterday of "a whole lot of nonsense about the personal lives of ministers" be enough to save him - with its plea for a focus on important things like health, education, crime and welfare reform?
Certainly it is hard to detect a consistent pattern in the interaction between sex and politics. The old cliche that Tory scandals centre on sex and Labour's on money no longer seems to hold good. Mr Cook seemed safe when his infidelities became public and, with indecent haste, he married his mistress, but his position looks a little more wobbly now.
The Agriculture Minister, Nick Brown, easily shrugged off revelations of his homosexuality, yet something altogether more hazy did for the former secretary of state for Wales, Ron Davies. Some, but not all, Labour sex is now scandalous too, it seems.
History is not much help here. Once, sexual shenanigans were kept kept quiet, unless, as with the Profumo affair, they were deemed to open the politicos to blackmail. He was sleeping with a woman who was also having sex with a Russian spy. Sleeping with the enemy meant something rather different in those days. Nor does it assist much to narrow the question to "Should a politician, when exposed as a philanderer, drop his wife or his mistress?" Cecil Parkinson was the modern prototype here. The chairman of the Conservative Party under Margaret Thatcher was revealed to have fathered a child by a rather formidable Tory secretary called Sarah Keays.
He dithered between the two, before finally plumping for marital loyalty with his dutiful wife, Ann, standing stony-faced by him. But it was a lose-lose situation - Ms Keays and her handicapped child were left her in straitened circumstance and Mr Parkinson was forced out of frontline politics.
By contrast, the Tory heritage secretary, David Mellor, seemed to have brazened out a much more lurid affair with an actress, Antonia de Sancha, and even more outlandish embellishments concerning toe-sucking and Chelsea football strips. The sex did not force him from the Cabinet. He only went after allegations that he took holidays from the daughter of a PLO fundraiser. But, after forcing the compliant Mrs Mellor to stand by him in a notorious family photo-call, he dropped her for an aristocratic departmental adviser. It was only after that that the voters of Putney issued their own reprimand by removing him from office at the last election.
Contrast that with Piers Merchant who, despite dropping his teenage lover for the second time and reconciling once again with his wife, was still forced out of the Commons by his own party officials.
The complicating factor in the political alchemy here was Mr Major's ill-fated Back-to-Basics campaign. It turned sexual morality into a party policy to such an extent that at one point the transport minister Steven Norris - known to his colleagues as Shagger - seemed under pressure, even though his six lovers were all, by and large, consecutive and all came after his separation from his wife.
So far, Labour has managed to avoid such confusion between personal and policy matters. Notwithstanding the contrast between his ethical foreign policy and his not-so-ethical private life, Robin Cook seemed, until this week, to have escaped whipping.
So has anything changed? The important thing to remember is that in politics it is not the presenting issue which is always the key determinant. The Parkinson case dragged on because he was a man who had many influential friends. Mr Mellor, by contrast, had few real heavyweight friends. With Mr Merchant it was not making a fool of his wife which did for him in the end, but making fools of his constituency officials.
There are other factors too. When there is not much other news about, it is quite possible for the press to get overheated about something which might otherwise be allowed to pass.
And then there are cases which become handy sticks with which to beat opponents. Tony Blair may have been right yesterday when he said: "One of the reasons people focus so much on books written about cabinet ministers by their former wives is because on the policy agenda there is not much criticism." But that does not necessarily make the vicarious criticism any less damaging.
Mr Blair may have been dismissive yesterday of a political agenda dominated by "scandal and gossip and trivia". But, as the example of the Clinton impeachment shows, politics often turns on things which are a good deal more arbitrary than mere facts.