What does Oedipus mean when he says "but Surely I must fear my mothers bed"?

When Oedipus says this, he has just been told by a
messenger that King Polybus of Corinth has died.  Oedipus has always believed that King
Polybus is his father.  As you know, the prophecy said that Oedipus would kill his
father and sleep with his morther.  Now that his father is dead and Oediups has not
killed him (he thinks), part of the prophecy is gone.

But
part of the prophecy still remains.  Oedipus could still sleep with his mother (he
doesn't realize he has been married to her, of course).  So on one level, Oedipus is
simply saying that his danger is not over because he still has to avoid sleeping with
his mother.

On another level, some scholars argue, we can
say that he is asking what would be so bad about sleeping with his mother.  He is asking
why he should fear his mother's bed.