Protect Children from Sex Tourism Act of 2013 - Amends the federal criminal code to: (1) eliminate as a defense to a criminal prosecution of transportation or travel in interstate commerce to engage in commercial sex act, proof by a preponderance of the evidence that the defendant reasonably believed that the person with whom the defendant engaged in such act had attained the age of 18; and (2) declare that the government need not prove that the defendant knew the minor engaging in such act had not attained age 18.