They had different sections at this camp for men and women. It was a matter of convenience for the camp, and it was another way to create psychological distress. Additionally, they knew that if they were separated from the women, that the men would not be as likely to fight back.
They also told the men to go one way and woman children the other, they killed all the children and most of the woman. this is why Elizer is told to lie about his age and say he is 18.