The central message of Who's Afraid of Virginia Woolf? is widely believed to be that human beings must learn to live without illusion. Throughout the play, the characters do battle to protect their own versions of reality, while tearing down each other's. In the end, however, all of the characters are laid bare to the cold hard truth of their lives.
Marriage is an illusion because it can easily dissolve once the people involved decide they don't exist anymore; divorce can be seen as the death of an illusion. In the case of Who's Afraid of Virginia Woolf? none of the characters have the chance to escape illusion since all remain married at the play's end.
George and Martha's relationship is made healthier with the exorcism of the imaginary son. Now they can face the world and each other honestly.