Disclaimer: Not mine. All Hail JKR.
It's been 10 years, and Hermione's been married to Ron for seven. Long enough to know she's bored. Long enough to know she's tired. And unfortunately also long enough to have had two small children, making it impossible for her to leave.
So she cheats.
It started during their engagement, with little things like sneaking an extra éclair when she was supposed to be worrying about fitting into her wedding dress. She wonders where Upstanding Hermione went as she takes stock of what else can be added to her list of transgressions:
He always says "I love you." She cheats with "You, too."
Every so often when they're supposed to see Harry and Gin, she cheats by begging off with a fictional ailment, dropping the regretful act as soon as Ron and the kids are through the Floo. Then she locates a book and a glass of wine, and spends some of her faux sick-time in the bath, pretending none of them exist for a few precious hours.
When the entire family's presence is required at the Burrow, she's snuck a Calming Draught or two (or 12). She debates internally if this one belongs on the list. It may fall more under self-defense against the wave of chaotic happiness one must endure there.
Oh- don't forget Owling Ron that she's stuck at work again. So sorry. She'll be home soonest, promise.
She cheats by trying to convince herself she's not depressed, and she doesn't need help.
One night, Draco Malfoy propositions her unexpectedly at a Ministry function, and she finds the answer surprisingly easy, as the instinct has been honed for years.
She cheats.