For better or worse it's just a fact that at the moment a girl working in a software shop is more likely to be a graphic designer than a coder. If I was in a rush looking for the new freelance designer one morning and walked up to an unfamiliar girl sat at a Mac Pro and asked her "Are you the new designer", I'm not being sexist, I'm just making an assumption based on the data I have. Designers and coders, whether male or female, both look the same: they're likely to be intelligent, smart and trendy looking and sat at a workstation.
Now, if in a similar situation I walked up to an intelligent, smart and trendy looking Indian freelancer sat at a MacBook and asked "Are you the new cleaner?", then that's totally different and inexcusable. I had enough data there not to make any assumptions, and it would betray racist core beliefs.
See the difference? Of course every situation is different, but I think the OP's example in this case was weak. We don't need everyone walking around on egg shells, paranoid that people are judging their every stated assumption against some kind of uber harsh politically correct scale.