The Supreme Court refused to hear the case, but that's not exactly the same as them not getting involved. It was a lower federal court that struck down the anti-marriage equality law in the first place. By refusing to hear the case, the Supreme Court effectively affirmed the ruling of the lower court. Refusing to accept the case is like saying that it is so obvious that the ruling made by the Court of Appeals (or Courts of Appeals, in this case) is correct, that they don't even need to hear it.
And, the voters can't defeat this. Their law banning same sex marriage was struck down as unconstitutional, and any new law banning it would be equally unconstitutional. In fact, now that same-sex marriage is legal, even the courts would have a hard time eliminating it. The SCOTUS ruling on Prop 8 said that it was unconstitutional to take away marriage rights in a jurisdiction in which they had previously existed. The only way outlaw same-sex marriage in those 30 states would be to amend the federal Constitution, which isn't going to happen.