'In this day and age . . .' - that's the phrase which got me thinking that, as sin, adultery is dead. That maybe we've undergone a sea change or turned a corner, that we are at one of those crossroads just before a social trend bursts out of the closet and the moral furniture is rearranged.
Perhaps Mr Mellor simply meant that he did not think his affair was a resigning matter because he believes his private life is not public business. Or that this particular dalliance would not affect affairs of state, as was seen to be the case in the Profumo affair, the grandaddy of all political sex scandals.
But I'm not really talking about David Mellor, except inasmuch as what public figures do - and what they get away with - is a barometer of public attitudes. Just four years ago, an affair with Donna Rice destroyed Gary Hart's presidential hopes in the US. This year, both Bill Clinton and Paddy Ashdown survived revelations of indiscretions. And so did Mr Mellor who, instead, blew it by taking those freebies. In this day and age, this appears to be a bigger offence than adultery, and maybe it is. Maybe a righteous God knows the character of real corruption in 1992.
Some people reckon the changes are merely the result of a cowboy press with the wraps off, but that's not all of it. Something's happening here. Some gut attitude has shifted. Affairs have always been a hard fact of life, but they were somehow something for the tabloids, something people did in foreign movies or France, in sitcoms and soaps, something Cosmo girls pondered, and women whispered about, like cancer. Adultery was on the margins, but now it's moved into the house.
Social changes - and, more importantly, changes in attitude - rarely register all at once, but in tiny jolts that go on resonating. Remember when nice girls allegedly did not do it before marriage? When their mothers said no man would marry 'damaged goods'? A polite new lingo has even evolved: you may bring your 'partner' (that politically correct, but oh, so sexless term) to an official event.
The same was true of people living together in what was known as 'sin', or unmarried women who had babies ('bastards' as they were called). Now about a third of all babies born in Britain are born to unmarried parents.
Suddenly you come around the corner and the world has changed: what was shocking is only ordinary. Do attitudes shift to accommodate reality? Is reality persuaded by perception? Self-styled moralists say we're moved by propaganda on the big screen and the box, where unmarried folk are always getting great sex.
What's going on here? Even the most cautious research shows that perhaps 40 per cent of married women and 60 per cent of married men have affairs - one-night stands, long-term commitments - and some figures are much higher. In official attitude surveys, however, 80 per cent of the population regularly says they disapprove of extra-marital sex. That means there is profound dissociation between what people want and what they do. Statistically, nearly everyone goes into marriage saying they believe in fidelity. Those who cohabit care even more. Rejecting the institutional supports of marriage, the emphasis is on a passionate and faithful relationship; otherwise, what's it all for?
The truth is a mess. Affairs contain every variety of human experience: ecstasy, misery, fun, guilt, love, horror. When attitudes towards pre-marital sex or cohabitation changed, it was as if someone opened the windows and the fresh air blew away the hypocrisy. With an affair, though, there's always an unwilling player. There is always a secret and, as someone said once, what good thing was ever secret? Someone gets hurt. Men get hurt. Women get hurt worse. There's still a double standard.
'I make the argument adultery is always a problem for women, not men,' says Annette Lawson, the author of Adultery, an Analysis of Love and Betrayal. In traditional terms, she's right, of course. In religious terms, adultery was only a sin for a man if he slept with another man's wife. Even in corporate life not long ago, policy dictated that if two people had an affair, the woman was fired. When Sara Keays refused to roll over for Cecil Parkinson, some considered her a thoughtless slut.
Affairs fulfil every cliche, and sometimes transcend them: the wife who stands by her man because she loves him or has invested in a way of life; the wife who colludes with her husband by never asking, and feels a sucker when she finds out; the 'other woman' who, after all the promises, finds herself empty-handed. There are also the odd couples who are actually happy. And, Ms Lawson's thesis notwithstanding, there are plenty of men who are deceived, betrayed and very badly hurt.
In this day and age, however, one way or another, an affair is no longer a 'resigning matter'. Something has immutably changed. There is a terrible ambivalence about it, though. The furniture has been moved and a lot of people are going to find it hard to find a comfortable place to sit down.