Why does Oedipus call himself "ungodly pollution"?

Oedipus is repeating the phrase used earlier in the play by the blind prophet, Tiresias. Then, Oedipus had expressed anger and indignation at such an outrageous description. But by the end of the play, when the full horror of what he's done finally hits home, Oedipus comes to accept that an "ungodly pollution" is precisely what he is.
The very presence of Oedipus in Thebes, this man who's killed his father and married his mother, has brought spiritual pollution to the city. Inadvertently or not, Oedipus has gone against the will of the gods. The consequences for him have been truly disastrous, but they could be even worse for the people of Thebes if Oedipus continues to stick around and pollute the city with his presence. The only way, then, for Thebes to be cleansed of this ungodly pollution is if Oedipus leaves, which he does.

Oedipus Rex (a.k.a Oedipus Tyrannus, or Oedipus the King) is an Athenian tragedy written by Ancient Greek tragedian Sophocles, presumably before or in 429 BC. In it, the Theban king Oedipus realizes that he has unknowingly fulfilled a prophecy which was given to him in Delphi, when he learns that he was the one who killed his own father, King Laius, and married his mother, Queen Jocasta. Oedipus is horrified of his sinful deeds and punishes himself by stabbing his eyes out, desperately begging to be executed. He hates himself for not listening to the blind prophet Tiresias, who told him that he was his father's murderer, and calls himself an "ungodly pollution," as he realizes that he has "polluted" Thebes with his sins much worse than the plague which has spread throughout the city.

Upon the discovery that he has murdered his father and married how own mother, Oedipus is disgusted with himself. When he refers to himself as “ungodly pollution,” this is both a revelation of the truth and an allusion to an earlier moment in the play.
When Oedipus was investigating Laois’s death before the elders, he calls upon the blind prophet Tiresias. In his speech before the elders, Tiresias calls Oedipus “ungodly pollution” as he begrudgingly testified that Oedipus himself is Laois’s murderer. In response, Oedipus becomes angry and accuses Tiresias of plotting against him.
Once Oedipus finally discovers that Tiresias told the truth, Oedipus uses the same phrase that the prophet had used to acknowledge Oedipus’ shame. This shows that Oedipus feels guilty for not listening to Tiresias earlier and deeply ashamed for the sins he feels he has committed. Oedipus pollutes Thebes and his family’s future because of his fate.

Oedipus refers to himself as "ungodly pollution" at the end of the play because he has learned that he not only killed his own father, but also that he slept with and had children with his own mother.  He is appalled and ashamed by his own fate, a fate prophesied to him, but which he has tried so hard to avoid.  Oedipus feels that he has offended the gods with his behavior, and so this makes him ungodly.  Further, he himself has become polluted by his deeds, and so he is no longer fit to live out the remainder of his life and be buried in Thebes, where he has ruled.  In fact, it was Oedipus who decreed that the murderer of Laius would be exiled, and so he accepts this self-bestowed punishment without conflict.