For all our fascination with sex scandals, it is rare for them genuinely to change history: rarer still for a woman's forbidden desire to be the cataclysmic force unravelling events. While politicians' peccadilloes linger in the memory, they are often of little public significance. Beyond the private hurt to the players and their families, little changes because of this dalliance with an actress, that fling with a rent boy. The personal rarely becomes properly political.
This is what makes Iris Robinson, an MP and the wife of Northern Ireland's First Minister, so unusual. Even if the fragile Northern Ireland power-sharing arrangement survives the caustic residue of her affair (at the time of writing, that remains unclear), by endangering it she joins a small and divisive group of women whose sexual transgressions ripple far beyond the domestic arena. An unlikely latter-day Helen of Troy but, perhaps because so few women still reach positions of real power, there are few modern rivals for the title.
Iris Robinson's affair with the 19-year-old Kirk McCambley is toxic because it breaks so many taboos. It isn't just the marital and religious barriers that she crossed - though those alone are difficult enough, in the puritanical milieu of Northern Irish unionism. Nor is it just about age - he a teenager, she nearly a pensioner - nor even about politics, the recklessness of a public figure using her influence to raise funds for his business. What is uncomfortable is that she transgresses deep-seated cultural expectations of femininity: that women are inherently risk-avoidant, and innately nurturing, and therefore able to comfort an unhappy 19-year-old without jumping into bed with him.
This was not just any teenager, but one Robinson had known from his childhood, a detail that transforms the affair from merely salacious to faintly disturbing. Like a teacher seducing a pupil, it suggests trust abused.
So it's tempting to follow some Irish media and question the labelling of her as "mentally ill". From the hysterics of Victorian times to the 20th-century women confined in Ireland's mental hospitals essentially for being sexually active, society has often marginalised transgressive women as mad. It reduces the threat posed to patriarchy by untrammelled female sexuality. It makes it seem reassuringly less likely that one's own wife might stray.
Does the diagnosis of depression reduce Robinson to an overly convenient one-off? It would be easier to accept her as a feminist poster girl for older women's repressed sexuality had she not damned homosexuality as an abomination. But the main problem with this analysis is ignorance. It is unwise to speculate on anyone's mental health, not least because sexual (and financial) disinhibition is a classic symptom of manic depression. Consequently, many on the left have taken her illness at face value and urged sympathy, suggesting that now is not the time for cynicism.
It is the humane and probably correct response. But nonetheless it is intriguing how readily we accept it. Would there be such understanding if Peter Robinson had been caught in flagrante with a teenage girl?
When womanising actors check into clinics claiming a compulsive illness, we snigger: sex addiction, indeed! Can't keep it in his trousers, more like. When middle-aged men in public life take mistresses half their age, depression may well be involved: sex can seem an antidote to the doubts of ageing, from physical decline to a stalled career and ominous sense of mortality. Yet such men rarely claim a breakdown in mitigation and, if they did, sympathy might be short-lived. A man's affair is still seen as a question of morality (for some) or opportunity (for the "Good on him!" tendency), not health. The mocking response to John Prescott's bulimia - which, taken alongside his own wilfully self-destructive affair, might suggest, if not mental illness, at least a man coping badly with stress - told us that.
The Robinson affair fits a pattern in which female transgressors are treated both more and less harshly than their male equivalents. We are more shocked, yet more willing to excuse it. Study after criminal justice study suggests female offenders are less likely to face prison, or more likely to get short sentences, than men guilty of similar offences. Yet, from Myra Hindley to Amanda "Foxy" Knox, female criminals often inspire greater public revulsion (and more column inches) than their male co-accused.
Iris Robinson killed nobody, but if her actions wreck a fragile peace process that has undoubtedly saved lives, then her failure - for whatever humanly understandable reasons - to put public duty before private desire is hard to accept. We may love the sinner, while hating that particular sin.
Celebrity sex sells
There is, meanwhile, one last awkward issue raised by this affair: that of privacy. The British press has shifted grudgingly away from the gleeful kiss'n'tells of the John Major "Back to basics" years: politicians are no longer outed for being gay and affairs are seldom revealed without some fig leaf of public interest. Wary of bringing the self-regulated party to an end, and aware that celebrity sex sells better than the dull political kind, the tabloids have invested less effort in Westminster sexposés.
This is mostly welcome. But identifying those few sex scandals with public significance - the MP who fires his blameless secretary to instal his mistress instead, or who swings state favours for her lover - requires raking through much seemingly irrelevant dirt. Iris Robinson is way beyond the tipping point at which the personal becomes political. But her downfall does raise questions about press scrutiny in more marginal cases. A legacy for which some colleagues will not thank her.