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.

Don't be too hard on yourself. By your own admission you assumed the sex of the author just because you have a wife who is teaching your own child. As far as my understanding of sexism goes this is miles away from being an example of sexism.

I was under strong impression it was post by woman before I even opened it: just saw the blog name.