Voyeurism...

Rachel and I stand outside Monica's door, silently pushing it open a little so we can see inside better, watching my sister fuck herself. Admittedly, the "strap-on" comment I think is what originally brought us both over here, I don't think either of us ever knew that Monica swung that way, or even felt that way occassionally. But as we stood there and watched her, I could tell both of us were getting really turned on: I was harder than dried cement, and Rachel was gently scratching her lower stomach, occasionally going a down to the crotch of her jeans and rubbing a little, while unconsciously playing with one of her braless tits through her t-shirt.

After a few seconds, I feel one of Rachel's hands on my cock. She begins rubbing it and squeezing it through my khakis, it's kind of obvious now that she wants to fuck.