It's okay to be privately surprised, as long as you think about WHY you're surprised (e.g. "girls don't code" vs. "there aren't as many women coders as you might expect given the # of women in the US"). The dick move is to belabor the point and treat the programming woman as a performing dog instead of a person.

Think of it this way: you meet a woman at a meetup about a technical topic, and you find you have projects/interests in common. If you spend the whole conversation interrogating her about how/why she learned to code, asking her questions about what her boyfriend/significant other/husband does or what he thinks about her coding, or asking for her "feminine opinion" on your consumer startup, you are a dick.

If you treat her as a PERSON, not a novelty, and talk about those interests that are relevant to the meetup, that's decent behavior.

It's about the coding, not the gender (stupid).