It will be “settled law” if SCOTUS upholds homo marriage and it won’t be over if they declare homo marriage unconstitutional.
If SCOTUS says there is no constitutional right to gay marriage, it will not overturn the gay marriage laws in the four states that adopted it by voter referenda, the 11 states (plus DC) that adopted it by votes of the legislature, or the four states where it was imposed by state courts under their state constitutions. It will also not prevent future votes by state legislatures to approve it where it is now banned (not that Oklahoma or Utah are going to do that any time soon, but places like Pennsylvania might.)