I always thought my girlfriend and her best friend were so innocent and sweet. I mean id done stuff with her that wasnt 'innocent' but other than that i had no idea how true the line 'Its the quiet ones to look out for' was. Anyway last year i used to work night shifts. My girlfriend J, 19, a cute fair skinned brunette hadnt long moved in with me. So i go to work one night. J says shes invited her best friend R around for a few drinks etc. I get to work only to find ive mixed my shifts up and i have the night off, so i head off on my way home (it takes me about 45mins each way) I bump into my mate on the train who asks if i wanna go the pub for a few pints, which i take him up on. A few hours later i get home. I go inside and no ones downstairs. I hear music playing up stairs so i slowly climb the stairs fantasies swirling about what i might find them up to till i get to the bedroom door. I look through the gap in the frame to see J sitting in the corner on the chair naked from the bottom down slowly rubbing herself. I think shes watching a dvd or something till i look through the opening into the mirror. In the reflection i could see R naked on top of her boyfriend on my bed. I should have went in and kicked off but for some reason instead i just watched with my hands down my pants. J just sat watching staring intensely as the fucked on our bed. I watched as R started to orgasm. Admitedly by this point i was yanking myself off pretty hard. Maybe thats why i didnt do anything while R's bf carried on wanking himself while he watched J do the same. It was hot watching them stare at each other while they played with themselves. Eventually he motioned her to join him on the bed, which she eventually did. She sat on the end of the bed watching him masturbate as she took her top and bra off. I was fuming but horny as hell at the same time. I was paralised between emotions so i carried on watching as she leaned forwards and took his cock in her hand. She started wanking him. R just lay naked watching as she played with her pussy. He signalled to her to suck it. She looked at R for permission. R just lifted her foot to the back of j's head and gently pushed her mouth over his cock. J took his penis in her mouth and sucked and licked him. This hurt to watch but made me hotter than id ever been before. I watched as she mounted his cock and started to ride him. R got up and put a leg over his face. She squoze his face between her thighs as J rode him faster. After what seemed an eternity j started to orgasm as he grabbed her hips and shot his load inside her. I went downstairs, finished myself off and went for a walk. I returned later to find her alone as if nothing had happened. We still havent spoke about it