LOS ANGELES — Within moments of the announcement Friday that the Supreme Court would hear two cases relating to same-sex marriage, gay activists rejoiced, heralding the decision as a major advancement for their movement.

‘’Today’s news is nothing short of a milestone moment for equality,’’ said Chad Griffin, a champion of the legal challenge to Proposition 8, the California initiative passed in November 2008 that banned same-sex marriage and is now heading to the Supreme Court.