The Amazon, not the rainforest, the warrior woman archetype thing. In ancient history they weren't portrayed any more sexually than warrior men, but then by the 1800's you've got authors writing books about their frenzied lust and whatnot and that became, like, the big thing about them.
Now, if fantasy or sci-fi has an all female group they are guaranteed to be portrayed in a way that's male gaze first and character second.