It is very dangerous to all parties to keep male and female Bettas together.
A male will stake out a spot to have his nest, and then he will defend it from the girls. This often means very, very violently defending it, resulting in the death of the female. This, however, is only one scenario. The presence of the male leads to competition between the females, who are trying to show the male that they are strong and fertile, and would make viable offspring. Sometimes the females will just flare at each other...and other times, they will fight mercilessly. This is why, when breeding Bettas, two females are not placed together with one male.
It is also possible to merely have an extremely aggressive male, who kills them regardless of whether or not he is "defending" a nest. On the other hand, it is also possible to have an extremely aggressive female, who kills the male or kills the other females.
Now, suppose they also bred in these numbers (it can and does happen). The male, after spawning, chases even the mother female away. Now he has EGGS in his nest, and suddenly he becomes a frantic, mildly psychotic father intent on keeping anything and everything away from his nest and his eggs. What chance do smaller (typically more docile, though not always so) females have against this suddenly hyper-aggressive male?