I hadn't been living with my slut ex Sarah long before I learnt what she was like. I got home from work late one night, and she was sat in the living room with her friend Rob, who I gathered was an ex boyfriend of hers. They'd obviously had a bit to drink, and I joined them for a couple of beers and a smoke. It being summer, Sarah was wearing a little white dress she had, and was clearly not wearing anything underneath it - periodically, particularly when the conversation turned a bit risque, you could clearly see her nipples harden and stick out, pushing tightly against the thin white linen. As midnight came and went, I said I had to go to bed as I had to be up for work in the morning. Sarah said she'd stay up for a while chatting, so off I went. Being a small flat, the bedroom was next to the living room, so I didn't have far to walk. I got undressed and climbed into bed. About 5 minutes later, just as I was dozing off, Sarah came in to say goodnight. She was quite drunk and stoned, her nipples were poking through her dress again, and as she kissed me goodnight, I slipped a hand up the bottom of her dress and copped a feel of her shaven pussy, which was definitely damp. "Later", she mumbled, and went back to the living room. I laid there and half dozed off, vaguely aware of the noise of conversation from the living room competing with the noise of the television. I suppose I'd been there about 15 minutes when I realised that the conversation had stopped, and a minute or so after that, I heard the bedroom door quietly open. I laid still, peeking through slightly open eyes as Sarah crept into the room and peered intently at me before creeping back out of the room again. I vaguely heard her say, "It's alright, he's asleep" before she crept back out again, leaving the door half open. As she went back to the living room, she switched off the hallway light, and then the living room light, and I heard the TV being turned up slightly before the sound became muffled - she'd pulled the curtain that hung across the living room doorway. I could guess then what was going to happen in there, but instead of being annoyed, to my surprise I found that I had a huge hard-on. Moving very slowly so as to make no noise, I crept out of bed, through the bedroom door, and into the hallway. I found that there was a small gap in the curtain I could see through into the living room, the glow from the television giving me a good view of everything that went on, while preventing them from seeing me there watching. Sarah was lying on the floor with Rob next to her, kissing her passionately. Her nipples were clearly hard, her small tits straining against the thin material of her dress as Rob's hands played wth them before moving downwards and rubbing her wet cunt, firstly through the dress, and then pulling the bottom of it up, exposing her pussy to the air and exploring it with his fingers. Sarah moaned and spread her legs wide to give him better access. She was also rubbing the front of his shorts, which bulged and barely concealed his own stiff cock. As I watched, she pulled the front of his shorts down, allowing his stiff prick to spring free beforeshe eagerly grasped it and began wanking it slowly. He shifted his position so he was kneeling by her head, the fingers of one had still playing with Sarah's cunt as he moved his hard cock next to her mouth. She wasted no time in leaning forwards and taking the tip into her mouth, teasing his helmet with her tongue as she worked his foreskin backwards and forwards. Gradually he leant backwards, still working his fingers into her cunt, and even above the noise of the TV I could hear the slopping sound her pussy was making. My hard on was in my hand as I stroked myself while I peeked through the curtain, knowing I should do something, but too excited to stop watching. After a short while, Sarah struggled to remove Rob's shorts, and they shifted positions as they got rid of their clothing. Rob tugged his shorts down, while Sarah stood up, and peeled her little white dress off over her head, standing there fully naked before him. She pushed him back down onto the floor before kneeling over his face, lowering her shaven cunt onto his face as she leaned forwards and took his cock into her mouth. Her sucking of his hard prick became quicker as she began to rub her cunt onto his face, her stifled moans showing that his tongue was bringing her close to coming, the movements of her arse showing that she was grinding her pussy into his face. Rob's hips began to jerk a little as he began to fuck her face, and I could tell that they were both close to cumming. Suddenly, she climbed off of his face, and turned herself around, one hand holding his cock as she guided it into herself, roughly jamming her sopping cunt onto it as she began to ride him. Her hips ground against his as she fucked him, and even above the noise of the TV I could hear the noise from her sloppy cunt. She leant back, sitting upright on his cock as he thrust into her, her using one hand to play with her hard little nipples, and the other to rub her clit as their movements became faster and faster. With a loud groan, she began to cum, and I imagined her juices gushing out of her as usual and running down his cock. Rob's hips jerked, and I could tell her was cumming deep inside my slut girlfriends cunt. I was close to cumming too as I stroked my cock and watched them through the curtain. Even after he had obviously finished cumming, Sarah was still having one of her long multiple orgasms, grinding herself onto his cock as she came over and over, her fingers and his cock keeping her there. Eventually, temporarily sated, she slumped forwards, kissing him on his mouth and neck before lifting her hips off of his softening, glistening wet cock. Moving downwards, she took his prick into her mouth, greedily sucking the mixture of his cum and her cunt juice off of it. Satisfied that she'd cleaned it up, she stood up, reaching for her dress, and at that point I sneaked back into bed to wait for her....