I once had sex with a female platonic friend of mine riding down I-85 in the back of a minivan... her parents were driving.

It was very very odd. We started cuddling, then somehow my hand went up her skirt, and then... into her panties... which then resulted in them being taken off. Much fingerbanging and handjobbing occurred for a while, and then finally- finding a way to look as if she was leaning her back to my chest and innocently cuddling- I took her from behind. It was somewhat uncomfortable, and I was convinced that I was going to be in huge trouble; it was only a matter of time before her parents glanced back there... but amazingly enough (I guess they were too tired, or maybe honestly didn't care) we pulled it off. She climaxed (both of us biting our lips so hard to not make any noise), and then finished me off with a handjob. She then licked the cum/vaginal fluid from her fingers, which astonished me.

We never did anything again... Hell, we never even talked about it again, and after 5 years I'm not sure if she would be willing to talk about it... but whooboy, it was very sudden, surreal, and hot.