mutual admiration, even from strangers, is sometimes what helps us identify with being a man. For me it all comes down to following my gut- that general sense about reading a stranger when you first see him- if you are (on a scale)
frightened by him,
repulsed by him,
neutral to him,
intrigued/curious about him,
attracted to him, or
feel like you have found your soul mate.
For me, as long as it's not the first two, If I'm not distracted by life's daily trials and I am *attentive* to what he's up to- I will often time 'be a pal' and let him look. No harm done. Why not? Especially if he switches hands to display his own goods my way. Obviously he wants to be seen, and to take a peek himself.
It's when I feel threatened or repulsed by someone that I will cover up and afford no private viewing of my body- because even though i am in a public/private place I can still control how much of my body is displayed and seen by whom.
It's the same way for me if I'm in the locker room at the gym. I'll uncover if I feel comfortable, or if it's clearly invited and returned behavior by someone I have a modest level of interest or attraction to (even on an emotional/mental level) but if I feel the victim of wandering creepy eyes, I show a towel and a scowl. That's it.