I Am Getting SOOO Pissed

Why are some people so stupid? Today while I was in the grocery store, one of the cashiers said "can I help you maam?" I finally snapped! I yelled at her as loud as I could "I am not a maam!" It really humiliates me when someone does that. My God, I know I'm fat where I shouldn't be, but for Shit's sake, I talk to people, can't you tell from my voice that I'm a man, or are you all just stupid?