My girlfriend says she's fantasized about people watching us have sex, and I have fantasized about other guys seeing her naked, about her stripping for them or skinnydipping--stuff like that--why does this turn me/her on?

for me, the thought that i am turning someone else on is very much a turn on for myself ... weren't we taught that it is better to give than to receive???

Probaly like me as in you love to see your g/f getting pleasure so the thought of other guys wanting your g/f turns you on even more
And she is comforable with her body and gets turned on seeing men wanting her

Its voyuerism. Perfectly natural. People want other people to see them

MY EX USED TO TALK ABOUT IT A LOT ENDED UP WITH HER MODELLING FIRST BIKINI THEN FULL NUDE OPEN LEGS SHE DID A COUPLE OF FUCK FILMS FOR A GUY AND WAS ALLWAYS CHATTING TO GUYS ON YAHOO NAKED IT JUST PROGRESSED SHE LIKED TO FEEL DIRTY AND BE NOTICED GAVE HER A EGO BOOST.
I USED TO BRING GUYS HOME AND SHE WOULD SHOW OFF IN A BIKINI BEFOR TAKING US ON AND SHE USED TO SHOW OFF INFRONT OF MY MATES .
SHE ENDED UP AFTER WE SPLIT (10 YRS) DOING STRIPPING AND LAP DANCING

It turns you on because it's HOT, plus somewhat taboo, which is HOT! Don't pass up opportunities when you and her are young and beautiful. Later in life, you'll either have erotic memories or frustrated regrets. Take her to a sex club, put up the chain and make like monkeys. The people watching from the other side of the barrier will definitely appreciate the show.