The reasons that Oedipus repulses us are fairly simple: he killed his father and he had sexual intercourse with his mother. In fact, he had sexual intercourse with his mother frequently enough to produce four children.
The reason that Oedipus fascinates us are more complex. On one hand, people are fascinated by sexual activity regarded as taboo, whether it is Bill Clinton's affair with Monica Lewinsky, Jerry Sandusky's sexual encounters with young boys at Penn State University, or the alleged sexual harassment accusations against Herman Cain. One can probably do an internet search and find numerous pictures and videos claiming to be sexual encounters between mothers and sons.
People are also strangely fascinated by horrific crimes. The 1989 murder of their parents by Lyle and Eric Menendez captivated Americans' attention. For some perverse reason, people cannot seem to look away when they hear of such terrible activities.
Of course, the most famous explanation for the fascination and repulsion with Oedipus was given by Sigmund Freud in his Interpretation of Dreams. Freud believed that it was the latent desire of all males to kill their fathers and marry their mothers. Freud thought that males wanted to kill their father because they viewed their fathers as sexual rivals for the affections of the mother.
One final and more tame observation: I would like to think that the story of Oedipus fascinates us because in Oedipus we see a person who is completely devoted to the pursuit of the truth, no matter what it's consequences. We are also fascinated by Oedipus because he actually discovers, during his lifetime, what the gods predicted for him all along. Apollo's oracle had said he would kill his father and marry his mother and that is precisely what Oedipus did. To come face to face with the divine assessment of who we truly are is a terrifying thought.