In our teen years my sister and I went through a phase where we were both very curious about sex. When I was sixteen and Melanie was fourteen, she helped me acquire a copy of an adult magazine from a friend at school. We cheerfully ran home with it and spent the afternoon looking at it, laughing at times, and admitting our own fascination with the stories and pictorials of others.
I soon became fully erect under my jeans, and she knew it. I asked to have the magazine for 'bedtime reading', but she refused and told me I would have to jack off in front of her because that's what she really was most curious about. I agreed on the grounds that she stripped nude and posed like the girls in the magazine. Locking the door to her room, we proceeded to do just that: Her naked and spread legged, showing me her pussy, pulling it open and fingering herself (she was an expert!) and me with just my dick out, stroking away.
We didn't last long: she came and while her hand was still clamped tight over her crotch, I reached my own climax and squirted my cum all over her bed. She was amazed by the process of ejaculation, even though she flinched when she touched it. We never actually touched each other. We did the same thing about ten more times, and then, less than a year later, never did that or anything else again.
Still, we recall those dirty little sessions we had fondly, though we are now both happily married and no longer desire to resume that brief part of our relationship.