Places I have done it before:
-In my car in a crowded parking lot with people walking by.
-On beaches, with and without crowd.
-In a darkened movie theater.
-In the community pool, at night.
-On a train at night, traveling across Europe.
-In my backyard, where all my neighbors can see me.
-In a castle ruin in Europe.
Places that is on my list to explore:
-In an airplane, I want to join the mile high club.
-In a football stadium, crowd optional.
-In a glass elevator high atop of the city, with a view to die for.
-In a store dressing room.
-In the men's room.
Any other suggestions?
