This happened when I was 19.
I went on a date with a girl I'll call Tina, my ex-girlfriend's next-door neighbor, coincidentally. She was short, dark and big-titted, with short dark hair. We were sitting in my car after our first date, parked with the front bumper of my car almost touching my ex-girlfriend's bedroom window, immediately beyond which she was probably sleeping. Thus I was nervous about her hearing us and looking out the window. My date found this amusing and began making occasional 'sex' noises as we made out. By the time her hand found its way to my bulge, it was a large bulge indeed.
It was hot, and I was wearing tight cut-off jean shorts, which made it difficult for her to get at my cock, even after I had unzipped my shorts, so I slid them down to my ankles. She then took my cock in her fist and began to pump very slowly while whispering dirty talk into my ear, sticking her tongue in my ear, and moaning.
After a bit of this I came; since she was gripping my cock rather tightly, the cum gushed out rather than spurted, erupting all over her hand; it then dripped all over my cock. To my surprise, she kept jacking me off, using my cum as lubricant. When her motions began to make wet squishy sounds, she increased her pumping speed. The squishy noises were funny, actually, and it made us laugh.
Not that I really believed my ex would look out the window, but the idea that she could made all of this more exciting. Soon, my cock erupted once again, making quite a big sloppy mess; my cock, balls, pubic hair, and thighs were sopping wet with cum.
When I masturbate today, I sometimes think about that double handjob in the parking lot.