I learned most of what I know about sex from spying on my sister
My older sister always had her boyfriend around whenever our parent were out.
She always made me promise to stay in my room and not to tell anyone about his visits.
I had the loft room which was directly above my sisters and even with my TV on I used to hear the noises through my floor. I was inquisitive to know what they were doing, so one day when I was in the house alone, I managed to pry up one of the floorboards in the middle of the room. I could see the cable to my sisters bedroom light and with the screwdriver I managed to chip away some of the plaster at the edge of the fitting until I could clearly see her bed below. After clearing up the dust from her bedsheets the hole was not obvious at all from below.
The very next time she had a visit, I carefully took up the loose board and eagerly watched the whole thing.
I knew that my sister was in good shape, but had not realised how good she was going to look naked and how adventurous she would be in the bedroom. From that point until she moved out, I lost count of the times I jerked off watching her perform.