Taken from ub:
Male bettas can be kept with groups of female bettas.
Reality: Many pet stores suggest keeping a male betta with a small harem of females, and some betta keepers have tried this set-up with limited short term success. However, in the long run, such an arrangement almost always results in the injury or death of the females, male, or both; male and female bettas were not made to co-habitate outside of brief periods of mating and will aggress upon each other. Male bettas should never be kept with any member of their species.
Myth: A male and female betta can be housed together as mates in the same tank.
Reality: Bettas are a solitary species; they do not form mate bonds and thus will not recognize one another as companions. The only time one should attempt to place a breeding pair of bettas together is during the actual mating; a male and female betta housed one on one is asking for a massacre. Even with a careful introduction and supervised mating, injuries and deaths during reproduction are not uncommon. Male and female bettas simply should not be housed together, period.