When I was 13, I spent half the summer with my grandparents in Mississippi. I was just beginning to discover sexual urges and felt a vague attraction to my cousin, a pretty girl one year older than I was. She was also staying at my grandparents that summer, along with her older sister. I used to look into their bedroom quietly every morning to see them in their pajamas.
At that age, I found just seeing them in bed was strangely stimulating. One night after I had gone to bed in the front guestroom, I heard noises outside my door in the living room. I looked through the keyhole of the old-fashioned door and could see my cousin and a boy. She was visiting with a boy a couple of years older than her and talking quietly. Soon they began kissing, 'making out,' and soon they got to petting. I was mesmerized watching through the keyhole as he began to rub her dress between her legs, slowly at first, then gradually more quickly. She gradually spread her legs and he massaged her vagina through her dress. I could tell from the look on my cousin's face that she enjoyed the stimulation. I became very erect and moved away from the door and onto my bed. I slowly rubbed my stiff penis and masturbated until I came. It was a wonderful sensation. It still makes me erect thinking about that episode 40 years ago.