One person in the marriage starts to lose interest. They have an affair. They ultimately leave (or are left by their partner when the truth is revealed).
But what if they didn't leave? What if the person having an affair keeps it going for the good of their relationship, all without their other half knowing?
That's what 'Cal', a 40-year-old mum of one from Cambridge, is doing. She believes that when she took the decision to begin an affair, it was the best thing she could have done to save her marriage. Which is why it's still going on...
Here, she explains to GoodtoKnow how her affair has made her happier than ever, and why she plans to keep her 'lifelong commitment' to her secret partner.
My husband and I met when I took over his local pub.I was single and working in and around the south east, and he came in one evening and never left!
In the beginning, things between us moved very quickly. Around three months after we'd met, my company changed hands and I had to give up the pub (or move to Kent), so we ended up living together at his parents, then got our own place shortly after.
It was great at the start, learning how similar we were and how many things we had in common. We both worked hard and enjoyed our free time together, but money was very tight and renting our house was really expensive, so we moved to a cheaper area and found new jobs.
This started off well, but as time went on he missed his family and seemed to resent me for moving us away (despite it being a joint decision). He'd call his mother every single day, in fact he'd speak to her more than he spoke to me. Then he started drinking more, because he was unhappy.
I tried everything to work through things, but he's stubborn and refused to acknowledge a problem.We had some counselling and he admitted that his job made him unhappy. Once he found a new job, things improved, but our sex life was almost non-existent. I talked openly with him, but he refused to change (or even try). He said that I needed to accept he'd never want sex more than a few times a year, but I needed it more than that.
I can still remember the moment I thought about, and then decided to have an affair.I'd read about Illicit Encounters on Daily Mail and thought it sounded like a fun escape. I just wanted to speak to a man who saw me as 'me' again, not as someone to take their frustration out on. I wanted to feel attractive and wanted again.
I joined and set up a profile, describing myself as the woman I missed, the woman I was when I met my husband. That's who I wanted to be.
The man I met is amazing.He's older than me by some margin and married, but has a grown family. He found his wife no longer wanted the intimate side of a relationship, and he missed the sex and passion. When the affair first began I felt like myself again, it was wonderful. I could laugh, flirt and relax again. I felt desirable, I felt attractive. I was happy.
The effect it's had on my marriage has been huge, and the way its made me feel can't be underestimated. I no longer feel downtrodden and worthless - I feel complete. I'm happy, and our marriage is happier for it. What I was missing at home I can now have elsewhere.
Am I ever tempted to leave my husband?In a fairytale, perhaps, but in the real world no. I love my husband and I love my family. Had I met my affair before my husband, it may have been different, but neither of us have the desire to destroy our respective partners. My husband has absolutely no idea about the affair, and I intend to keep it that way.
My affair is lifelong as far as we're both concerned. We've exchanged our own private vows to remain faithful in our affair; we're in love when we're together.
To those that may criticise my choice, I'd say that every situation is different.What works for one person may not work for another. This works for me and has saved my marriage - I don't feel that I'm cheating because my affair gives me something I simply wasn't getting at home. I tried to find it in my marriage, and my husband refused it, so I found it elsewhere.
I'm not doing anything in my affair that I'm denying my husband, he made a choice and I made vows to stay with him. This makes it work for me, and I'm not hurting anyone because my affair is a very closely guarded secret.
Would I recommend it? Some may not be able to handle the guilt associated with an affair. If that person can separate sex and emotion, then an affair can work and I would definitely recommend it.
Relationship expert Dr Pam Spurr isn't convinced that an affair is the answer to marital troubles...With all the will in the world people can swear they won't let an affair get out of hand, that it's good for their main relationship, and it will all be okay. But unfortunately reality hits in so many ways and the affair they think they're in control of starts to control their life.
What happens when one or both people in the affair fall for each other at an emotional level? It's very hard to separate out the emotional connection. Yes, some men manage to do this, and some women too. But many end up staying in the affair for more than sexual reasons.
People are convinced they'll be extra cautious and won't get caught but that's an incredibly naive attitude. People get caught all the time and are often shocked at the utter devastation when an affair is discovered.
I always ask people who say they want to have a 'harmless' affair to imagine what would be like if the shoe was on the on the other foot. How would they feel if their partner was entering into a so-called harmless affair? Wouldn't they feel pretty devastated? Do they really want to go behind their partner's back? Shouldn't they be thinking of ways to improve their relationship?
There will be those who get away with their affair, who keep it under wraps, and tidy it away into the past. They think it's actually helped them stay in their main relationship.
But it's far more likely the affair with change how they think about their actual partner and often they go looking for more excitement rather than trying to rekindle things in their relationship.
You can follow Dr Pam Spurr on Twitter and her website