For me it was one of the hottest things ever ... so much so that I felt compelled to make this my first post here ( a belated hi guys).
A few years back my g/f at the time and I went hiking on a woods trail and ended up sneaking off the beaten track for a quickie. After a minute or so I saw out the corner of my eye we were being watched by a fellow hiker. Once I got over the initial split second shock it I realised I enjoyed him watching me continue to pound her pussy hard from on top.
I hoped she didn't see him as I was afraid of her reaction but before long the other guy moved closer and began to rub his cock from outside his jeans. I felt her tense up when she saw him and as she was about to push me up I whispered in her ear that it was making me horny and all but begged her to let me keep going.
I have a sneaking suspicion she got off on watching him get his cock out of his pants and begin to jerk off while watching the two of us. She never told me so but I let her multiple orgasm that felt like it was going to rip my dick off do the talking for her.
Ever since then it has been something I have gotten off over.
A friend told me a similar story, but from the other viewpoint. He was at a parking lot at a park in Philly -- a place along the river where couples go to fuck in their cars. (Why do these lover's lanes always have great views? The people don't look at the view anyway! LOL). He walked up and stopped many feet back from a car where a couple was fucking so they wouldn't see him. But the guy looked up and caught him. Just then the watcher put his finger to his lips in the "Shhhhhh" sign. The guy fucking didn't tell the girl they were being watched but raised himself up and started taking longer strokes so the guy watching could see his dick going in her pussy. He kept looking up to see if the other guy was still watching and the girl asked "What are you looking at?" (the windows must have been down for the other guy to hear this?) and he said "Nothing."
This same guy also told me a story about a street (also in Philly) near a bunch of bars where people sometimes fuck in their cars with people they pick up in the club. He watched as a guy fucked a girl then robbed her including her necklace and kicked her out of the car and drove off. Which just goes to show you can't be too careful who you jump in a car with.
During Mardi Gras in New Orleans I have seen couples fucking in their cars many times. I think they meet someone on Bourbon Street and both the guy and the girl are sharing a hotel room with 3 other people so they have to go to one of their cars for "privacy." But it is in a public lot so people walking by can see them if they aren't drunk out of their minds, which eliminates a lot of people. LOL. But as someone previously said, at least it's strangers who see them and not their friends so I guess they feel that is better.
On vacation in Montreal, I was up on Mount Royal overlooking the city and there were people fucking in their cars there also.