If men are from Mars and women are from Venus, then it would
be several decades before someone created the space travel technology for men and women to finally meet.
What’s common here? Male is usually the default, but females rule is gradually becoming more common and more obvious. The lack of the other sex is flashed briefly; Disaster wiping out the other half, or voluntary separation are two common reasons, although sometimes it seems they just don’t appear.
See on io9.com