A year ago, the Environment Minister's head might not have rolled. Cabinet ministers have successfully steered themselves out of broken marriages. An injudicious subsidised holiday, not adultery, finally killed David Mellor's political career. The Transport Minister, Steven Norris, has survived accusations of promiscuity. Cecil Parkinson's demise might have been avoided but for the fury of Sarah Keays - and even he was subsequently rehabilitated.
Neither infidelity nor the fathering of a child outside marriage fully explain Mr Yeo's downfall. The fatal blow was his perceived betrayal of a key government policy on single parenthood that has seduced the party faithful but left most voters cold. He had a bit part in the attack on single mothers; but as a member of the Government, Mr Yeo bore a collective responsibility. 'Family values' was conceived as a clever way to unite a party riven by conflict over Europe. It appeased party activists who were also angry at the deposing of Margaret Thatcher. It was hijacked by the right at last October's poisonous Tory party conference.
Ministers on the left of the party stayed silent or even - and Mr Yeo was among them - colluded with this pulpit politics. So when one of their number flouted the new orthodoxy, sympathetic ministers could not prevent his resignation. Virginia Bottomley and Emma Nicholson - women conscious of life's realities - offered support, as did Douglas Hurd and Norman Fowler. Their lack of influence shows the gulf between liberal Tory MPs and hardline activists.
Too late, John Major has recognised that he let the 'family values' campaign evolve from a sober statement of common values into a quasi-evangelical mission. Recent interviews suggest that he wants to temper the message. But senior right-wing colleagues have a different agenda; and now like- minded activists have demonstrated that they can depose members of his government.
Tim Yeo was a middle-ranking minister of no great consequence. But his demise should serve as a warning to Mr Major against placing party unity above his natural instinct for common decency.