The problem with "clear state interest" is that it's an ambiguous term. I could say that there is no clear state interest behind allowing the unborn to be killed through legalized abortion because I don't recognize that some people think that fetuses aren't people/human and thus don't deserve the legal right to life (just as some who oppose SSM can do so based on their religious/personal opinions regarding what is and is not a marriage). Abortions and SSM aren't the same issue nor am I trying to link them, but what I am saying is that "clear state interest" could be anything, the people of the state could believe that homosexual marriages destroy society or the sanctity of marriage (which is stupid reasoning) but in the eyes of a state putting the measure to ban or legalize gay marriage on the ballot, that can qualify as state interest (especially when it is approved by a majority vote by people in a state).
If gay marriage is to be legalized federally a constitutional Amendment must be ratified that does so in my opinion. I don't think it's within the US constitution to give the federal government the power to overturn the majority of state Constitutions on the issue when the US Constitution isn't clear regarding sexuality and marriage. I would love to use the Constitution to say that abortion should be illegal because it violates the constitutional right of human persons, but the concept of "person-hood" is too vague in the Constitution for me to definitively say that with a legal sense and thus legally ban abortions under how the Constitution currently stands.