For the most part, the Greeks did not believe in a different afterlife for the good or bad—i.e., no heaven or hell. In their view, the afterlife was almost universally grim; the important detail for the dead was whether they were buried or unburied. Those who did not receive proper funeral rites were doomed to wander by the river Styx, the entrance to the Underworld, for eternity; their souls could never be at rest. Thus, denying burial to a corpse not only insulted the body, but also damned his soul for all time.
The buried were granted access to Hades, the name of both the Underworld and its king (who was also known as Pluto). In order for the dead to gain this access, a complicated ritual had to be performed. There were few ‘professional' undertakers, so a man's funeral fell to his family, especially the women of the family. They prepared the body for cremation, oversaw the collection of the bones and ashes and burial of the urn, provided the tomb with liquid offerings (libations), and led the mourning, a loud and violent process in which women tore their cheeks with their fingernails, ripped out their hair, and poured dirt over the heads and clothing.
Mourning the dead was one of the few things women were allowed to do in ancient Greece, especially Athens. Women of well-born families were expected to stay at home in specially designated women's quarters at all times except during certain religions festivals. Marriages were arranged by a girl's father or guardian. Women were not true citizens of the democracy and could not speak or vote in the assembly. They were not even allowed to speak in court, a basic right for Athenian men.
Burying and mourning their dead relatives gave women an opportunity to do something important for their families. It brought women to the fore and gave them a role to play. When Creon forbids burial of Polynices, he denies Antigone the chance to do one of the few important things society allowed women to do. Thus, he is attacking her identity, and that is a large part of the reason she opposes his orders.