Matou Kariya and Tohsaka Aoi were friends since childhood.
Even after Aoi married into the Tohsaka family, they stayed friends. Kariya met Aoi and her two daughter frequently.
After Aoi gave birth, her husband Tokiomi stopped to sleep with her, so Aoi grew more and more sexually unsatisfied.
At some point it finally happened: Aoi and Kariya slept with each other. What happened once, happend twice and soon they started to sleep with each other behind Tokiomis back.