From my experience, I see that usually hackathon events have more male attendees than female. I am sure there might be hackathons where that's not the case but I don't think it would be very inviting for any female to go to an event that's full of the opposite gender. Just imagine if you were one of 5 males in an event of 100 attendees and the rest were females. I think you would be a bit uncomfortable/out of place even though that's not the case.