Often I will wake up in the middle of the night snuggled up close to my girlfriend. We both sleep naked so I get turned on easily and, not wanting to wake her up, I will quietly masturbate in bed next to her.
We are open about our masturbatory habits but it is still a great turn-on for me to think she might wake up and 'catch' me...moreso, what will she do?
She has woken up to me masturbating a number of times and each time is different. One good story is when she rolled over asking me what I was doing and resting her head on my chest, slid her hand down between my legs and rubbed her finger on my perineum and ass, allowing me to continue and finish in spectacular fashion.
Another time she woke up she rested her head down low on my stomach and grabbed the base of my cock as I stroked in order to point it at her face when I came.
There are many similar stories. Good stuff!