I'm sorry but none of this is sexist. Making an assumption about someones gender, race, age, etc based on what is most common (or most common to you) does not make you a sexist, racist or agist. It just makes you human.

If, after realizing your assumption was wrong, you said "omg, I can't believe a woman knows haskell, how did she ever learn that?" THEN you're being sexist.