This story is about my sister's butt. I was 13 at the time and she was 16.
My sister started doing the 20 minute workout from TV, late 80's. I just ate cereal and watched. Mostly un interested until this one day, all changed. As I was looking at the tv, I started getting hard. I never cared to look at my sister normally but she had these short shorts on and was all legs. I started staring at her ass. A couple of my friends thought she had a sexy ass.
I couldn't take it any more I went to my room and jerked off. Did this a few days in a row, stared until I was super hard and went to jerk off in my room.
Then one day the tv was messing up, we had rabbit ears if anyone remembers that. My sister was pissed and she asked for help. I got up to mess with them, she was reaching up and got behind and was reaching too, my cock brushed against her ass. She froze, I didn't know what to do. So I ran to my room. She chased after me. She asked if I enjoyed myself, I said not really missed the workout. She said rubbing my cock on her ass, I said I dunno. She turned around and said do it again. I got up hugged her from behind and press my cock on her. She laughed, started moving it up and down a little, I started dry humping her. She said STOP!
She pulled her shorts down, pink undies, no try. I began again. The feel of her undies and the way she tightened her ass around my cock was too much, I grabbed my cock through my pj hole, and two strokes I shot all over her undies!