From The Worst-Case Scenario Survival Handbook: Dating and Sex by Joshua Piven, David Borgenicth, Jennifer Worick
Pick a rendezvous time.
Select a time when you are least likely to have to wait in line and when you will not be disturbed. The best times are just before the plane reaches cruising altitude or during the in-flight entertainment.
As the plane is ascending, listen for a beep from the in-flight messaging system.
The first beep comes without a subsequent announcement and indicates to the flight attendants that cruising altitude has almost been reached and that it is safe to begin their preparations. The FASTEN YOUR SEATBELT sign will still be illuminated, but the flight attendants will get up. As soon as the flight attendants clear the aisle, head for the lavatory. Try to select one that is not visible from the galleys. Have your date wait at least a minute, then meet you in the lavatory. You should hear the beverage carts roll by. After a few minutes, the flight attendants will begin to serve drinks, blocking the aisle from passenger access. Alternatively, or in addition, proceed to Step 3.
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Meet during the movie.
Plan your rendezvous for the beginning of the film, preferably when the film is at least fifteen minutes underway. Most passengers and flight attendants stay out of the aisles and galley areas during the entertainment portion of the flight, so you will have more privacy. You should proceed to the lavatory first, to be followed a minute later by your date.