Just like any other interfering passer-by, I asked her if I could help. In between the sobs and the tears, it turned out that her boyfriend had cheated on her. A few more questions (yes, I am so damn nosy) and I discover that she and loser boy had what he thought was an open-ended relationship.

Open-ended. And now definitely ended.

Affairs are bad news. You know it. And it’s hilarious that when politicians are on the nose – as so many of them are these days – we get the full-scale assault on their private lives with all kinds of tittle-tattle about extramarital affairs.

In the dying days of the Howard government, there was quite a lot of gossip about the then-prime minister (none of it ever, ever proven and, in my inexpert view, highly unlikely). Already, social media is filled with rumour about the state of the Abbotts’ marriage (completely ridiculous. Margie Abbott is a saint and Tony would be mad to think he could ever do better).

I’ve had a few of these conversations about infidelity over the years. Distraught woman discovers her partner is cheating on her. Sometimes it’s just a misunderstanding (she was feeling more serious about it than he was but they never got around to resetting the parameters); sometimes it’s downright deceit (he had two parallel families in neighbouring suburbs – it’s amazing how long it took him to get caught); sometimes one of them is actually looking for love and thinking an affair will fix it.

Somewhere in the middle is a vast group of women sobbing in bathrooms; and men sobbing into their beers. Or just drinking a lot of them very quickly.

It’s not always the men. Women are not always angels and men are not always nasty pieces of work. I reckon these things happen because people refuse to speak to each other about what really matters in their relationships. And because monogamy has somehow become old-fashioned.

Which I do not understand.

Why is it that people who don’t have enough time to cook for themselves or do their own shopping or even wash their sheets regularly suddenly think they have time to embark on another relationship as well as the one they are currently in.

How is it that this most complex of human endeavours – the negotiation of getting to know someone, having a relationship with them, getting to know their bodies so it is truly enjoyable to have whatever kind of sex you both like, making conversation beyond the mundane – is suddenly so easy you have the time and the capability to conduct two at once?

Anyhow, the young woman in the QVB bathroom and I had quite the chat. Turned out that she hadn’t made it explicit she was serious about the young man in question. And she hadn’t ever made it clear she would be pissed off if he decided to have sex with someone else.

I suggested, in that hideous and overbearing way that older women may sometimes have, that if she decided to have another relationship, it might be useful setting some boundaries about acceptable relationship behaviour. Off she went, still bleary-eyed but the snot and the mascara were wiped away.

Tammy Nelson, author of The New Monogamy, says cheating doesn’t make us happy; nor do we often end up with the people with whom we did the cheating.

According to her book, about 10 per cent of affairs last just one day (you know it, that party where you wandered off and had a shag in the spare bedroom with someone whose name you don’t even know. You bump into them 10 years later and think, gee, you look familiar. Very familiar.) Only 3 per cent of men marry the women with whom they have had affairs; and the divorce rate among those people is 75 per cent. Hard to trust someone if you’ve had a bad beginning.

Which makes me wonder whether those in relationships are always looking for someone better. Is infidelity really about looking over the shoulder, hoping someone with more and better prospects will come into view?

Or is it just because we have no resilience when it comes to the relationships we are in already? Do we treat our relationships as disposable – move on to something new if the smallest thing goes wrong? Does monogamy seem like too much of a commitment?

I know that some brave souls decide they will remake their relationships after an affair – it takes some doing to overcome the deceit and the betrayal and the pain. It also takes a helluva long time to rebuild trust when it’s been abused.

And there’s just one way to avoid it in the first place – and that’s to make it perfectly clear to all concerned that part of the deal is monogamy. That’s not because you are trying to be selfish and keep all that adorable scrumptious hotness to yourself.

It’s this. Show me a person who says they can manage two complicated relationships at once and be loving, open and available in both relationships. And I’ll show you a liar.

Twitter @jennaprice or email jenna_p@bigpond.net.au