Valhall-
This is where the law is grey. Some see the similarity between professional licensing, some see polygamy, while others, like myself, look at current
practices. My marriage must be recognized in all other states without regard to my hetero status - it stands to reason that because my marriage has
already set precedent - gay marriages would follow. The differences in interpretation set in motion the wheels of litigation. Hence the courts and
eventually SCOTUS will have to weigh in.
From an ACLU archive:

The ACLU believes that anti-gay marriage laws violate the "Full Faith and Credit" clause of the U.S. Constitution, which states: "Full Faith
and Credit shall be given in each State to the public Acts, Records and judicial Proceedings of every other State." The idea behind this clause was
to make sure that in a nation where people could freely move from state to state, each state would respect each others laws. In other words, final
court decisions of one state -- including Hawaii's -- must be recognized by other states.
Moreover, the ACLU believes these laws violate equal protection, since they seek to discriminate against lesbian and gay Americans based on
homophobia. The laws also violate the right to interstate travel under Supreme Court precedent, which has held that a state cannot discriminate
against people entering its territory by imposing unconstitutional conditions on the right to enter.

It is because of Full Faith and Credit and guarantees under the 14th amendment of "equal protection" that I forsee this ending up in SCOTUS. There
is too much at stake here with regard to these two sections of the constitution.
This is why I think to circumvent the courts you will see a concerted effort to pass and ratify a constitutional amendment on this issue.
B.