The NYT carries a poll today showing how various groups feel about gay marriage:
To me, gay marriage is more about signaling than it is about any sort of legal or moral principle. On one side, Democrats want it given approval as an official signal of America's progressiveness. On the other side, Republicans resist because allowing it would signal America's endorsement of a lifestyle with which Republicans (a sizable part of America, after all) do not necessarily agree.
This debate skips right over the more fundamental question of why any marriage is in the state's prerogative in the first place, however. Marriage is essentially a contract, but unlike virtually any other contract, the terms are written by the government. Yet why the government is thought to have such a special competence in marriage contracts that it should be the sole author is something I've never heard explained.