Even more fundamentally, where in the Constitution does the State impart to itself the authority to conduct a marriage? This was the province of the clergy.
There is nothing whatsoever in the US Constitution that gives the federal government the authority to impose homosexual marriage on the States. Regardless, I would be very surprised to see the SCOTUS rule against the homosexualists. Certainly, several justices (probably a majority) will firmly avow the government’s authority to do whatever it pleases in all matters. It’s not like they haven’t done it before.
BTW, there’s nothing stopping the homosexualists from amending the US Constitution to expressly protect homosexual acts, but why bother? Five justices is all they need to do whatever they want. Civil society, democracy, and the rule of law be damned.