My husband cheated on me with his ex-wife, and I'm trying to understand why.

I need to try and understand why my husband would cheat on me with his ex-wife of all people. I know it’s not because he isn’t over her. He is. I believe that. It must be about something else because it just doesn’t make sense.

Why cheat on me with someone you’ve had sex with for over a decade?

Why bother using up your one chance to cheat with someone we can never be rid of?

It would have been better if he’d slept with anyone else. I wish it had been a really hot girl, or a stranger, just anyone but his ex-wife. This woman is my nemesis. She hates me. Now she has the satisfaction of knowing she has one over on me, and he allowed it to happen.

He says she seduced him.

He was looking after their children one night and woke up to find her naked in bed next to him.

“She started it, I promise. I know I should have left, but I didn’t.”

I started howling. I screamed and dropped to my knees sobbing. The betrayal I felt was overwhelming because of what he’d done but also because of who he had done it with. I knew the ripple effects pf this would be felt for the rest of our lives. We would never put it behind us.

It was worse that he chose to cheat on me with his ex-wife, so much worse in so many ways, in terms of the effect. However when it came to trying to find a reason why he cheated on me, it helped in some sick way. There would be a real cause, a solid reason, something aside from boredom and attraction. Maybe it was because she was familiar, because they still shared a bond, because it was comfortable or comforting.

It wasn’t because there was anything wrong with us and its not like she’s a threat to me. He left her years before I came along.

While I can forgive him, I can’t forgive her. All those years I spent trying to be nice to her, to understand why she hated me, to treat her with respect, were wasted.

Now I hate her just as much as she hates me.

Check, mate.

I’m lying in bed trying to let it all sink in. He’s at work going about his day as though his world hasn’t been completely shattered like mine.

Strange thoughts keep occurring to me. I’m remembering the movie Single White Female when Allie’s boyfriend Sam cheats on her with his ex-wife. She finds out after overhearing a desperate phone message the ex-wife left for Sam and they fight and make up. I was young at the time and I remember thinking, “I’d never forgive my partner for cheating on me with his ex. Never, ever, ever.”

I’m thinking of our first meeting, what he said when he explained his first marriage, how he described their relationship today.

“What does she look like,” I’d asked.

“Incredibly beautiful,” he had said.

Last night on The Good Wife Alicia slept with her estranged partner Peter and afterwards when they were getting dressed they were talking about how “sexy” the sex was, and wondering why. Alicia said, “Because sex is always sexier without love.”

Is that it? Is that why he did it?

I’ll probably never know.

My best friend thinks he had sex with his ex-wife because he secretly wants her back but I know for a fact that isn’t it. He had plenty of opportunities to go back before he met me and he never did, so why would he do that now?

Another reason why some men go back for sex with ‘the ex’ is closure. This actually rang a little true for me. While they’ve been apart for years, they never really had any closure.

Then there’s guilt. I know better than anyone how guilty he feels about the failure of their marriage, the hurt he caused his ex and the hurt he caused his family.

And, he didn’t start it. I keep going back to that. He’s not the one who seduced her. She seduced him.

In the movie The Big Wedding the cheating gets complicated. Article continues after video.

At least now we have an opportunity to set some boundaries between them. Gone is the trusting, complicit little girl who never protested at getting left behind while they spent time together as a family. No more sleeping over at her house when she is working late and he is looking after his kids.

From now on there will be a complete separation between our lives and our families. It’s the only way I can move on and he’s agreed to do whatever it takes to stay together. There really is no other way.

For everyone who thinks they’d never forgive their partner for cheating, think again. You never know what you’ll do until it happens to you.

I love my husband and I know he loves me. He made a huge mistake but he did it out of weakness. His ex did it out of malice.

I will never let her break us.