Just once with a complete stranger. Was stuck at the Minneapolis airport during a terrible blizzard. All flights were cancelled and the airport was packed. I was lucky because I'd managed to get one of the last rooms available at the airport hotels. Went to one of the airport restaurant/bars to eat, drink, and generally kill some time. Restaurants were packed too, since no one had anywhere to go, and everyone gets hungry.
I had an empty chair at my table, and offered to let an unaccompanied redheaded girl sit with me. We just started talking - not flirting or anything, just two bored people eating nibbles and drinking to kill the time. After a few hours of this, we just knew what we ought to do. So, we went to my room. It was mind-blowing, and we both seemed to do it because of sheer boredom. I don't even remember her name.