Serious answer from someone who's in the NYC group:

in practice, people do care. Both guys and girls are more likely to attend an event with a less skewed gender balance. Gender balance helps attendance at events where there's socializing.

I don't know what kinds of topics you're imagining, but usually we don't focus on topics that need to be single-gender. Our topics are things like clustering algorithms and prediction markets and TED talks. Nothing you really need to shoo the girls or boys out of the room for.

Not that I wouldn't mind a Rationalist Hen Party sometime.