I was in a band in my twenties, traveling a lot, and my friend and fellow musician was engaged to a very sexy, very sweet lady in her early 20's. One night after our gig he had to go home for the weekend, and he asked if his fiance, I'll call her Mary, could stay with me for the two nights. My hotel room had two beds, so I figured ok, and I had no intentions of messing with his wife to be. She was already in bed when I got out of the shower, and with the lights off, I just jumped into bed naked, with a robe handy for the morning. Although I was very aware of her in the room, I slipped into sleep, but awoke some time later to a soft moaning sound. I came instantly awake, hoping that she was really masturbating over there!
My cock was immediately hard, and I gently moved the covers over to the side so I could rub myself as I listened to her. Her breath would get faster, then she'd moan very quietly, and I could hear her hands moving under the covers. I got wet quickly, and as I slid my hand up and down my shaft, the slippery wetness made a squishing noise. She got quiet for a minute, and I kept on, hoping she'd hear me too. I tried to be as quiet as I could, but when I exploded, I'm sure she heard me, and I heard her come very soon after. The next morning, neither of us said anything about it, but she kind of blushed and smiled when I asked her how she had slept.
We never did actually acknowledge to each other that we had done that, but it's a memory I'll never forget as long as I live.