I’m a little too old and too much of curmudgeon to answer your question; but I’m not to old to off a suggestion on how to search this message board.
I found several entries for different threads. Oddly several of them seemed to be from around 2003.
The World According to Garp has a similar cautionary scene, and the car where the action was taking place was parked in a driveway when the crash happened.
I’m surprised no one has mentioned it given there was a hit song about it 54 years ago…Under the Boardwalk
Up against a statue of Queen Victoria.
If there are multiple bathrooms on the plane and if it’s a relatively quiet portion of the flight ( no lines for the restrooms ), I imagine this would be pretty easy. Person 1 gets up to go to the bathroom. They enter the bathroom but don’t lock the door- they stand inside by the door with the door open a crack. Person 2 notices which bathroom they walked into and gets up a minute or two later and enters the same bathroom.
The only possible complication happens if another psssenger on the plane gets up and heads to the toilets during the interval between the time Person 1 and Person 2 leave their seats. That why Person 1 has the door open a crack. If Person 1 sees someone who is not his friend approach the bathrooms, he closes and locks the doors for a few seconds, thus diverting the passenger into the other toilet and “clearing the coast”.
As long as you do this during a point midflight when people aren’t lining up for the bathrooms, it’s easy and you won’t get caught. Other people may head for the bathrooms during your tryst but if there’s another unoccupied toilet they aren’t going to notice how long the other toilet has been occupied.
Then, when you’re done, you have to check and make sure the coast is clear and there is no one in line before the first person exits the bathroom. Then the second person can exit a minute or so later.
If you time it incorrectly and find that there is an actual line for the bathroom by the time you’re done, then you both have to exit together and you’ve kind of been busted. But it’s not like you’re actually going to get in trouble.
The key is finding that quiet point on the flight - not right after the meal, not right before landing, when relatively few people are heading for the bathrooms. Not difficult on a long flight.
Not that I have an experience with this 
Back room of the West Texas convenience store where I worked.
An empty guard booth on the campus of a small agricultural college in the Lower North of Thailand.
On a darkened outdoor stairway in central Bangkok (back in my salad days).
And much to my then-rommate’s disgust, on top of his freshly cleaned suit that he had laid out on the bed but my lady friend and I failed to notice.
Wow, really old thread being revived.
Let’s see. At least for me:
A college library in Paris during my study abroad years. That section was dead empty that Monday, and made for a great and thrilling time.
Public restrooms in general. Once you get in a stall, and as long as you don’t start screaming out your sex partner’s name, you should be fine. I did it probably about 5 different times that I can remember. One time was on a work trip to Saudi Arabia. That was a risk. If we got caught, I don’t know what would’ve happened to us. 
The car, although only fun if it’s comfortable. An ex of mine and I had sex in a Yugo when we were visiting the Balkans. It was cold as crap and we were freezing. Not to mention there was barely any room. I had to sit outside the car to catch my breath for about 30 minutes afterward.
A quiet park also works well. I went on a trip to San Francisco with my boyfriend, and we visited this park overlooking the city. It was literally deserted, so we went for it.
I’ll post some others if I can think of them.
Whatever you do though, don’t get caught by anyone who holds a grudge against you; if you do, you’re fucked.
Wait…
Maybe in a particular bar or three back in those days too. Ah, my lost youth.