Otho will not let you fight in the arena if your character is a woman. However, if you are male, he will let you fight increasingly difficult enemies. After these are completed, Otho will tell you that nobody is brave enough to fight you. He also sets up the fight between you and Benny, if you choose to kill him that way. A female courier will be able to fight Benny in the arena.
There is a special low Intelligence dialogue option with him, in which the Courier challenges Otho to fight instead of requesting him to set up one.
pc If you speak to him after completing Et Tumor, Brute? he may say "A woman actually proved useful for once. Caesar was bold to put his life in your hands" even if you are playing as a male character.[verified]