When parents divorced while my mom worked an out of town job we lived with her father. He was very strict, I was not well behaved, and she left disciplining me up to him. That usually consisted of over the knee/over the lap spankings. Those gradually evolved into longer and longer sessions, the barehanded spankings stinging a lot but not brutal. I often became erect from them. He realized I was more or less grinding against his leg as he spanked me. When he asked me outright if that was what I was doing, I admitted it. This evolved into protracted spanking sessions that mixed spanking with him f******** my bottom and fondling my g******* from behind. He never asked or forced me to do anything to him, even though i think i would have if he had asked. This went on for some months before the first time as I lay across his lap in one of these sessions I heard and felt him open his pants, and felt the heat coming from his p**** as he masturbated and I felt him shoot warm on my bare cheeks as his other hand cupped and squeezed my testicles and stroked me until I shot as well. Eventually it became a routine practice every few days when my mother was gone that I would simply come up to him and drop my pants and lay across his lap voluntarily. That evolved into him having me completely undress for the sessions. I was early teen when it started, and it is something that I voluntarily continued well into my 20s.