I had another experience at an AMP in a small town where I lived at the time. The masseuse I usually saw there often massaged me "undraped."
On this one occasion, a lot of people were coming in without appointments, and she kept excusing herself from massaging my back and hips, to go greet the drop-ins and ask them to wait.
About the third time she did this, she just left the door to my massage room open while she went out. I continued to wait face down on the table, completely naked, and with my ass in full view from the doorway.
She must have told a couple of young ladies to wait in one of the massage rooms, because I heard them coming down the hall and chattering.
I also heard the two of them stop at my open doorway, go silent for a moment, and then burst into giggles. They moved on after a moment, but I guess they liked the view, because they went back and forth several times, still babbling in a foreign language I recognized (but don't speak), and giggling.
While they were doing this, I felt both embarrassed by the exposure, and turned on. As I felt my cock getting hard against the massage table, I fantasized about turning over to show these young ladies the rest of the view, or at least looking up at them, but I did neither. Though there was a substantial community of people speaking that language in that town, they speak English, too, and the town was small enough that the odds were, I knew one or both of them.
Remembering them seeing me naked (though only face down), and fantasies about inviting them in to wait with me, always turn me on when I masturbate by lying face down in the nude, and pressing my cock into a towel on the bed.