Legal Question in Constitutional Law in South Carolina

It shouldn't. The Supreme Court is going to decide whether there is a fundamental right to marry someone of the same sex. If the answer is no, that won't mean same-sex marriages are illegal. It would mean states can choose whether to allow or them or not.

The Court has not been asked to decide whether the Constitution forbids same-sex marriage, at least in these cases. That's a very different question, and the answer is pretty clearly no.

If the Court says there's no fundamental right to same-sex marriages, then some states that currently allow such marriages might stop. That should not invalidate marriages they have already allowed, though I suppose it might allow the states themselves to invalidate those marriages if they want to.

The Supreme Court sometimes does surprising things, so it's possible that will happen here. But the Court is supposed to only answer the specific questions raised by the parties. No plausible answer to the questions that were raised here would forbid gay marriage.