The California Supreme Court will rule on Tuesday on a challenge to Proposition 8, the ballot measure that re-instated California's ban on same-sex marriage.The court is considering a series of lawsuits that seek to overturn a constitutional amendment approved by voters in November which overruled a court decision that briefly legalised same-sex unions. It amended the state constitution to declare only marriage between a man and a woman was valid or recognised. It followed the court's 4-3 ruling in May 2008 that declared the previous marriage law violated the rights of gays and lesbians to marry the person of their choice and was discriminatory. The ruling made California the second state, after Massachusetts, to legalise same-sex marriage. The lawsuits claim the proposition was put on the ballot improperly. The court also will decide whether the estimated 18,000 gay couples who married before the measure passed can stay legally wed.