This happened a long time ago so please don't judge me.
I forget the exact reason but I was looking after a girl from the neighborhood at her home. I was about 17 and she must have been 14.
We were watching TV and she was lying on her stomach on the floor in front of me while I sat sideways on the sofa doing homework.
The girl was wearing a short school jumper and white panties underneath. I know this because the way she was facing I could see up her skirt. After a few moments, she ran a hand under her tummy and lifted up her skirt. I watched as she slid a finger into her vagina and worked it in and out while rocking her hips against her hand. When she got closer to coming she got up on her knees, with her butt up in the air and moaned while fingering herself. I did not think she remembered I was there.
This was too much for me and I took my penis out from my pants and quickly jerked off while watching. When I came I spread sperm all over my pants and my notebooks.
After she was done, I just sat there, with my books covering my wet crotch and pretended I had not noticed. The girl acted like nothing had happened either. When she left the room I got up and went to the bathroom and wiped up.
Later I thought that she had done this deliberately and wanted me to have sex with her or something but I never was brave enough to find out.
I still think about that day when I masturbate.