My best friend was screwing my exwife for a year before I screwed her. We started dating serious but he never stopped screwing her. He never asked my permission, when he wanted some she gave him some. I watched occasionally but most of the time I slept in the guest room. It was common for me to get up early and make coffee and wait for him to get him some before he starts the day. Everyone within our circle of friends knew they screwed and I got off on it.