i've read an article on such stuffs quite sometime ago. can't
find the link anymore.
most probably, you're doing this because it makes you feel good or
better about yourself because you possess something, in a way,
highly desirable by other men. the idea of them getting aroused by
your girlfriend gives you the kicks. sort of like a esteem
booster.
it's healthy to have fantasies, but do a reality check once in a
while.
anyway, i'm new here. hello 
