So for celebrity watchers it was a bolt from the blue when the nanny, 26-year-old Daisy Wright, revealed how she and Law, six years her senior, had a passionate affair earlier this year while they were in America and Law was filming All the King's Men with Kate Winslet.
According to Ms Wright, the affair started after she and Law took one of his children to see the rock star Robert Plant in concert. The pair sat up late together, drinking and talking. Ms Wright admits to being a little drunk, and they ended up in bed. Once she had gone public and Law could no longer deny the affair, he apologised, very publicly, to Miller. "I am deeply ashamed and upset," he said. "I want to publicly apologise to Sienna and our respective families for the pain that I have caused. There is no defence for my actions which I sincerely regret."
Here is a man who has no intention of baling out to set up home with the nanny. He clearly loves and values his fiancée. And more than that: this is a man who was coming out of a long marriage that had produced three children. He knows all about families and responsibility, so simply falling in love doesn't tell the whole story. He made a decision to marry Miller.
What on earth was he doing risking it all? Yes, the nanny was pretty, and yes, she was available, but couldn't he have resisted temptation? What exactly was so irresistible about the nanny trap? And is it really as endemic as all that?
According to Tricia Pritchard of the Professional Association of Nursery Nurses (Pann), there's not as much nanny/husband sex going on as we think there is. "I've worked with Pann for 20 years, and probably dealt with only four cases like this," she says. "It would be naive of me to think there weren't men out there who think the nanny is there for them as well, but in most cases it's a professional relationship and the nanny becomes a family friend. She's there to make the children's lives more stable, not break up the parents."
Ben Black, director of Tinies, a national childcare agency, supports this view. "The nanny/boss thing is a bit of an urban myth. I run a chain of 20 agencies, placing about 300 nannies a year each, and in the past five years there's only been one case."
Cases do crop up - such as that of Carol Mead, 39, from Hullbridge in Essex, who told a TV programme how she confided in her trusted nanny Katie that she was worried her husband was having an affair. She later discovered through e-mails that it was Katie he was having the affair with. After a confrontation, the husband left with the nanny and Mrs Mead was left to bring up their three children alone.
And then of course there are the famous ones: British ambassador to Washington Peter Jay fathered a child with his live-in babysitter Jane Tustian. Singer Chris De Burgh had an affair with the nanny while his wife of 17 years, Diana, was in hospital. And now Jude Law.
So was Law simply living the cliché, taking the chance for a bit of attachment-free sex, or are there more complex issues at work here? Wright describes how Law had told her how wonderful he thought she was. How similar their outlooks and parents were. "I said to Jude that I didn't understand why he didn't find a wife who didn't want a career and to party all the time. He said it is very hard to find a woman who wants this, and that he would love that more than anything, but there aren't women like that in his line of work. He said I was very special to want that."
Chartered counselling psychologist Leila Collins of Middlesex University specialises in complex relationships. She says: "A lot of men with busy working wives see the nanny as being more willing to look after him than his wife is, just like mum did. There's also the question of power. Perhaps he feels more powerful with the nanny than he does with his fiancée who is earning good money and getting a great deal of attention."
And in the case of Jude Law and Daisy Wright - he a film star, she a "mere" nanny - that power struggle doesn't have a cat in hell's chance of being equal. Tellingly, Wright confessed how when Law first kissed her she thought: "I cannot believe this. Jude Law is snogging me."
Another issue may be the blurring of the roles when another woman is let so intimately into the heart of the family. Relate counsellor Christine Northam says: "For some men, seeing someone else looking after his children can result in confused boundaries about sexual desire and motherhood. And when those boundaries are muddled, an affair can happen very quickly. It can be catastrophic. As a counsellor, I'd say look at the chaos and the consequences that result from letting those boundaries down. It's all about immaturity and losing control, really."
Jude Law must be thinking about just that at the moment, and about yet another enduring cliché - how losing control in a wine-fuelled moment of desire can lead to a lifetime of regret.
Hey Jude, avoid temptation, get a Manny
By Julia Stuart
One solution to preventing your husband from running off with the hired help is to employ a male nanny. Or a manny, as some call them.
Jean Birtles, director of Top Notch Nannies in London, has 10 men on her books. "I'm very keen on placing male nannies because there are so many children in Britain who do not have a father figure," she says. "They have that extra energy; they're very good at kicking a ball and racing on their bikes. And they're brilliant at cleaning."
Troy Pearce, 23, from New Zealand, has been nannying since he came to London two and a half years ago.
"You can lead by example and teach children skills and things to help them," he says. "The view of a lot of people is that a male is more sporting and they'll rough and tumble with the children. Some people think you're a paedophile because you work with children. But most people assume that you're their father."
The times he most enjoys with children are when he takes them out on trips. "I think male nannies have a different style to women. I'm more likely to be more active than watch them play. The best thing is when the children are laughing or they're doing something that you have taught them to do."Reuse content