I remember one day when I was working on the register at my old job, and we weren't really that busy, but for some reason I can't remember I was a little flustered and hurried, and I wanted to finish puting a couple of customers through so I could get back to whatever business it was. Seeing them come up the entrance walkway at a glance, I went back to fumbling around and asked 'So that's one adult, and one child?' and a woman's voice said, 'I'm not a child!' and I looked up and to my horror it was just a normal couple!
I don't know how it happened, but I'd somehow in my haste mistaken that poor woman for a child! A little, tiny girl! It was ridiculous and I apologised profusely and everything was fine, but my point is, cashiers are hurried, and they see so many faces come through every day, that when taking a look at a person, they don't even really see them. I'm sure it was the same for you.
And hey, that's when you go 'I'm not a girl', and they'll apologise and learn from their mistake, goodness knows I did!