Imagine a friend hears the tale of Oedipus Rex, and says, “Oedipus should still have known better and not married!”: how would you respond?
How would you respond, knowing what you do about the ancient text?
Your hypothetical situation presents an unrealistic option for Oedipus. He saw no reason not to marry Jocasta. According to the prophecy he discovered at Delphi, he was destined to kill his father and marry his mother. Because Oedipus believed his parents to be Polybus and Merope, the king and queen of Corinth, he fled that city in an attempt to thwart the prophecy. From Oedipus' perspective, he succeeded; his flight from the city guaranteed that he would neither kill Polybus nor marry Merope.
The Greeks believed that the gods felt no obligation to be fair in their dealings with humans. Even though Oedipus himself had done nothing to warrant his doom and his efforts to prevent the fulfillment of the prophecy were futile, his actions were foretold, and he could not change them.
He is offered Jocasta, the queen of Thebes, as part of his reward for solving the riddle of the Sphinx, and he sees no reason not to marry her. He believes that he has run away from his fate.
The friend might as well as why Laius and Jocasta, Oedipus' parents, had children when they knew that their son was destined to kill his father and marry his mother. The Greeks who saw this play understood that they simply had to accept the will of the gods. For more explanation of this perspective, I recommend that you read E.R. Dodds' essay, "On Misunderstanding the Oedipus Rex." (I've included a link below.)
Secondly, because Oedipus solved the riddle of the Sphinx and freed Thebes from the plague, the citizens named him King of Thebes. Jocasta was already queen, widowed because her husband Laius had been killed "at a place where three roads meet." How could Oedipus decline the position of king? Greek culture had strict rules about hospitality; refusal of a gift was not acceptable. Therefore, Oedipus had little choice in accepting Jocasta as his wife. Again, however, the prophecy had foretold that Oedipus would marry his mother. He is so proud of himself for having solved the prophecy that he must have felt invincible. For that reason he's confident that he can eliminate the plague Thebes is suffering at the beginning of the play.
Although your question is based on a hypothetical situation, it is highly unlikely that Oedipus would have made any other choice. He believed that his mother was Merope, the wife of Polybus, King of Corinth; therefore, he fled that city to prevent the fulfillment of the prophecy. Thinking he thwarted the prophecy by not marrying her or killing Polybus, Oedipus had no fear about marrying Jocasta. He thought he could marry whomever he pleased. Alas, his pride was his downfall.