My sister, Jackie was 15 when this happened.
Jackie was never really into sports at school. She bunked off whenever she could and used to tell me that the staff never seemed to know who was on their period and who wasn't. She used that as an excuse a lot...UNTIL a new head of department joined and within a term all the old PE teachers had gone and were replaced with a far more astute team.
Jackie, like many of her peers found themselves underdeveloped (for gym, anyway) and unfit. So, about half way through the term Jackie told me that they were doing circuits and the thing she couldn't handle was the rope climb. I offered to help as I love climbing and gym in particular. I set up a rope in our yard hanging from a huge oak tree. Jackie eyed it nervously, but said that it looked safe enough and that she would have a go. She went indoors and came out in her school PE kit. Airtex t shirt, shorts, trainers. I told her to go ahead and show me what she could do... short answer? Nothing. She couldn't get herself off the ground.
After about an hour or so, I got her to sit on my shoulders, and showed her how to wrap her feet around the rope to act as a brake and a point of leverage and then, very carefully, I moved away. This left her hanging on the rope, but at least she didn't fall. Then it was 'Let your knees part and bring your feet up near your ass. Grip the rope with your feet and stand up.' It was only after a couple of these that I noticed a little dark spot appearing between her legs. Then she was about 2/3 of the way up and it was too high for her. So she gently let herself down. Of course, the rope was right between her legs and bumping along on her pussy as she very slowly let herself slide down. I noticed her neck go bright red as did the top of her chest. She all-but fell the last few feet and I caught her. She grabbed me and I heard 'Huhhh. Huuuuuuuuhhhhhhh' before looking down to see that what started as a little wetness was now running down her legs.
She ran indoors and I followed. I knocked on her door and said 'You ok?' She was crying so I opened the door and went in. She was on her bed with her shorts, (soaked) on the floor. Her panties were soaked through too. As I sat on the bed, she flung herself into my arms. After a while she calmed down and we talked. Yeah, she had cum on the rope, but what I didnt know was it was her first ever cum. Her room smelled of sex and I had a boner fit to burst. Not that my sister turned me on, but well, wet pussy is wet pussy right? Jackie reached for me and soon was jacking me off and when I came I spurted on her.
I am 24 now and Jackie and I still masturbate each other sometimes. She DID tell me she told her friends that rope climbing is REAL good fun.