I have come across this discomfort with pumping in front of others in
Australia, where breastfeeding in public is much more culturally
acceptable.  It is my impression that women feel uncomfortable about
pumping in front of others because it is not a "natural" activity
like breastfeeding is.  A mother breastfeeding is, to many, a
beautiful sight that provokes a warm glow.  Pumping is a mechanical
activity that is functional, but not inherently beautiful.

In fact a number of women I have spoken to have expressed a distaste
for pumping at all, saying they feeling like a cow on a milking
machine (particularly double pumping).

Ros Escott
Tasmania, Australia