We had a small group of women that had been victims of very traumatic violence done to them by men. If they had been put into a class with men right away it would have been tragic. We had a very good female instructor at the dojo that worked with them.
After about a year of working with each other and having my son (aged 14 at the time) enter the group at a time when they felt they could handle it, they gradually entered the regular mixed practice. As they got desensitized and then resensitized to having men around in the practice they all got to the point where they could function. Only one stayed for a few years, but all of them felt that their training was beneficial.
I think the training should be mixed as much as possible.