What is the 'problem of Susan'?

The 'Problem of Susan' is the fact that all the other children from the Narnia books, and their parents, died in a train crash. They were immediately invited into Aslan's Country, which is an allegory for paradise. Susan was not killed, and not allowed to enter. It is suggested in the books that Susan is too concerned with lipsticks, nylons, and invitations to parties to be allowed back into Narnia. Instead, Susan is left all alone in the world and forced to cope with the deaths of her whole family. This seems like a very cruel way to punish her for such a small thing. It seems as though she is punished for her female sexuality, which Greta refers to when she mentions the 'sin of Eve.'