That's not our fault, that's merely a product of the fact that hackathons are mostly guys. If it were mostly girls and only a couple guys, then the reverse would happen when guys showed up.
We shouldn't have to pretend that it's not weird for the one girl who shows up to what was an all-guys event. We should figure out how to get the gender ratio closer to even in the first place so that it's not a big deal when girls show up - no pretending necessary.