What drives people to cheat?
It was said of Angie that she "ran off" with her boss. It was said of Henry that he "ran off" with his assistant. It was never said of either of them that they simply fell in love and established a new life together.
Here's why: Henry, 43, was married, with three young children. Angie was 15 years younger than Henry and keen to have children of her own. Julie, Henry's wife, was humiliated and devastated by an affair that became a divorce.
Angie's story is hardly new: "I never intended any of this to happen. Henry and I worked very closely together in the firm where he was a partner. Almost from the beginning, I realised I was in love with him.
"I never would have said anything, even though I realised he wasn't happy in his marriage - you can usually read the signs. He hated getting calls from his wife at work, and they never seemed to have much of a social life together. I think he felt he was unappreciated at home.
"The first time he told me how he felt about me, I was really confused and disturbed, even though I was flattered and delighted as well. But I warned him what would happen if we had any kind of affair. He said it wouldn't be an affair, and he was right. It was the beginning of a whole new life for us."
Here's Henry's version: "When Angie arrived on the scene, she lit up my life. She was a joy to work with - bright, witty, terrific with clients, and very beautiful. I couldn't wait to get to work each morning. Things had deteriorated at home, long before I met Angie. All the intimacy had gone out of it - all we ever seemed to talk about was the kids and our domestic arrangements.
"I have no hard feelings toward Julie and, of course, the kids come first. I move heaven and earth to spend time with them on my access weekends. Of course it's hard not seeing them more often, but Angie is hoping to get pregnant soon. That's our plan."
Julie sees it all quite differently: "I'm no psychologist, but I think the technical term that best describes Henry is 'rat'. Once I found out about his affair, I couldn't wait for him to be out of my life. There's no way I could live with a man I no longer trusted, even if I had been able to forgive him, which I couldn't.
"The whole thing was such a cliché. Wife frazzled by the demands of three children under seven. Husband flying high at work, getting home later and later. Adoring assistant batting her eyelashes at him. Henry simply lost interest in me, not just sexually, but as a person. I pleaded for a bit of understanding - a bit of slack - but he'd stopped listening.
I think he really thought it should all have stayed the way it was before the kids arrived.
"My girlfriends have been wonderful. They won't let their partners go anywhere near Henry. I think they're afraid they might catch his disease. I tell them infidelity isn't a disease, it's a character defect. Henry was gutless, deceitful and reckless. He destroyed everything we were working towards."
So why would anyone do that? Workplace affairs are no mystery. Many people are at their most attractive at work - better dressed, better organised, more amusing, talents on display, not burdened by domesticity. It can be a very different picture at the end of the day, when partners are tired, and the demands of family life take over. ("I go to work for a break," say many harried parents.)
Unfaithfulness can sometimes be a reaction to the gap between the thrill of love's pleasures and the grind of love's work. Marriage is tough; parenting is tough; running a household is tough. Infidelity is often a sign of a person who hasn't learnt to bridge that gap.
Men and women can be addicted to sexual conquests, sometimes driven by narcissism, sometimes by insecurity. Some people crave excitement in their life, and our society makes sexual adventuring easier than ever. Straying is positively encouraged by new mobile phone apps that stimulate the fantasy - and facilitate the practice - of trawling for casual sex.
Unfaithful partners sometimes claim they were driven by such intense passion it was beyond their control, or that it was a "meaningless" fling (though the betrayed partner may feel the meaning was crystal clear). Others insist that monogamy is not natural - forgetting the other "natural" urges we curb in the interests of decency and fidelity.
The greatest gift any of us can give a partner is the peace of mind that flows from knowing they can trust us. But there's more to infidelity than sex. We can be unfaithful to a partner by withdrawing emotional support, or by letting our work get in the way of our marriage, or by living in ways that are not true to the values we claim to espouse.
And that's the real point about infidelity - sexual and otherwise. By betraying our own values, we're ultimately cheating on ourselves. No wonder most infidelity ends in tears.
- Hugh Mackay's new novel Infidelity ($30) is published this month by Macmillan.
- Daily Life