Hang on, that's a little unfair. If you work in an office, and you know they have hired one person, and that is a cleaner / designer, walking unto the new face in the office and saying hi you must be the new cleaner / designer is not a prejudicial assumption. Especially if they are sitting in front of Blender / a mop.
No-one mentioned "seeing an eastern european with a mop". To expand on my analogy, if you met an eastern european/etc. at the company Christmas party, asking "so are you a cleaner?" is presuming negative stereotypes based on race/ethnicity.
My point is that implicit in your statement is the claim that programmers are superior to designers. I think this belief is both more commonly held and voiced on HN and itself worse than the stereotype that women are more likely to be designers; one is a statement of superiority of a group of people and the other is (without the first assumption) a nonjudgemental statistically reasonable assumption.
Is it really a negative stereotype that women and men are equally capable but that a woman is more likely to be interested in an equally respectable artistic career over a technical one?
It seems that people on HN hold this developer superiority belief so strongly that if someone asks "are you a designer?" they think that is actually an offensive statement. How dare you think I am a plebe designer and not a developer god that I am?