From the U.S. Constitution, Article IV, Section 2, Clause 1: "The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States."
From the Pennsylvania Consolidated Statutes, Title 23, Chap 17, Sec 1704: "Marriage between persons of the same sex.
"It is hereby declared to be the strong and longstanding public policy of this Commonwealth that marriage shall be between one man and one woman. A marriage between persons of the same sex which was entered into in another state or foreign jurisdiction, even if valid where entered into, shall be void in this Commonwealth."
From my interpretation, if one state recognizes same-sex marriages, then the Constitution says that this PA law is Unconstitutional.
More thoughts later.