Answer:
In Things Fall Apart, the customs that Achebe portrays coexist because of strict gender roles.
Because men and women in Ibo society have strictly defined roles, everyone is kept "in place." Men, who are the dominant members of this society, have all the power and therefore control the law (the judicial system).
Women are viewed as weaker and their roles in society reflect that status. The only respect Igbo women were given in this society came through their connection to the divine (the priestess).
Because the structure of Igbo society was very rigid, people rarely left the roles society wanted them to play. (That's why Okonkwo hated weak men.) With the masculine and feminine sides of society separated, the two were able to coexist (especially since the masculine side held all the power)
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