The idea is pretty much like this:
She married him at a very young age. Sex was boring at best and also very rare. She never enjoyed it and in the end divorce was the ending of such marriage. Later on she's with someone else, her ex however never got around the divorce. She's happy, he's fairly miserable. Things change after a bit, he finds someone new and is happy, he has also changed and is a much better self. The ex meets him by chance and is surprised by it. Soon after the ex and her love have a horrible discussion and she thinks that it's all over.
The story starts that very same night. She calls her ex, crying and sobbing, and he goes to her home to comfort her. And at that point she starts to kiss him all over.... then something else has to happen. Possibly very violent and non consensual sex, in the end she will call him again, so that he keeps on cheating and on having those violent encounters. Then we will see where it leads us!
If you're interested then please PM me, don't answer here. No subject and one line requests will be ignored.