California's gay-marriage ban is again being tested as a high-profile federal-court case opened in the state Monday and immediately drew a ruling from the U.S. Supreme Court.

The high court temporarily blocked the Internet broadcast of the trial, the first serious challenge in federal court to a state law against same-sex marriage.

The main issue in the case, being heard in U.S. District Court, is whether a 2008 voter initiative dubbed Proposition 8 violates the U.S. Constitution by creating a law that discriminates on the basis of sexual orientation. Defenders of Proposition 8 say it validly defined marriage in traditional terms by restricting marriage to people who could have children naturally.

Whatever the outcome in Chief Judge Vaughn Walker's courtroom, the case will likely be appealed, eventually to the U.S. Supreme Court. Five states currently permit same-sex marriage, either by statute or under their state constitutions.