I moved in with my girlfriend while she was living with her sister. I kinda was attracted to her sister but figured it would just cause way too much trouble for everyone if I ever did anything about it.
It was something I thought about a lot, though.
I remember one night when I was staying up late, I heard clear sex noises coming from her sister's room. I was surprised because I didn't think she'd brought anyone home. The next day I asked her a little naively if she'd had someone over. She shook her head, confused, and it suddenly dawned on me that I had heard sex noises, but it had been her on her own.
I'm not talking a few moans. She was yelling and shouting and sounding for all the world like she was having the fuck of her life.
I was embarrassed and excited to think that she was so noisy when she masturbated. I made a bit of a habit staying up late on my own, and while most nights I didn't hear anything, once or twice a week I'd hit the jackpot.
The loud parts only lasted for about a minute. I figured she only got noisy when she was about to come.
I was turned on listening to her, and desperately tried to think of a way I could watch her while she was hard at it, but (probably just as well) never managed to.
I did often take my cock out in anticipation of hearing her. I'd go at it slowly until I heard her, then I'd speed up and try to race her to orgasm. She almost always won though.