When I was about 15 I used to work as a Saturday girl in TKMaxx. When I had my initial training on the fitting room we were advised what to do in this sitaution, simply state "Men to the Left, Women to the right" and then you could make up your own mind, it saved anyone any embarassment. I was amazed how many times I actually had to say it. The bigger stores do have huge communal changing rooms, but the male and female sections are normally on different floors, I'd say Sherry1 has an excellent suggestion, take a female friend along and no one will bat an eyelid.
Happy Shopping x