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You lean against the wall in the darkened room and enjoy the show. Kelli is now sucking on Tina's nippes and Tina's moans are getting louder. Tina has two fingers in Kelli's soaking pussy and Kelli's muffled moans are really exciting you.

You think "What the hell" and whip your dick out and start stroking it

Kelli is now laying back on the the sofa and Tina has her face buried in her best friend's pussy. After only a few minutes Kelli yells out "OH GOD! I'm gonna cum!"

Someone grabs your cock from behind you and whispers in your ear "Shall we join them or make our own fun?"