First of all not everyones marrige is tied to a religious faith.

It is one of the reasons civil unions (state santctioned marriges developed, in addition to other legality issues.

Under the Comity Clause of the Constitution, the public acts of one state must be recognized by other states and this principle has long been established in U.S. law.

Article 4 - Section 1 -of the U.S. Constitution declares:

Each State to Honor all Others

Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.

It played a majior role in finally bringing about the acceptance of inter-racial civil unions in the past despite the opposition of the majiority.

Why not the provide the same acceptance for same sex civil unions as well?

The answer is found in two little words.

Fear and bigotry.