Well, if we're gonna be 'classy.'
The most erotic thing I ever saw was on a plane waiting for takeoff. Everyone was in their seats bored and for some reason I sat up to look around. A woman with long hair a few rows in front on the other side of the aisle sat up too, lifted up her hair with both hands exposing her neck, and did a big stretch. I think I was the only one who saw it and she didn't know I was watching her. Don't know why it had such an impact but it drove me wild.