The court is scheduled to hear arguments March 26 in the legal challenge against Prop 8, which California voters approved in a 2008 referendum.
The ballot initiative came in reaction to a California Supreme Court ruling that laws banning same-sex marriage violate the state Constitution. Proposition 8 amended the state Constitution to abolish legal same-sex marriage.
Two couples involved in the U.S. Supreme Court case say Proposition 8 created a "badge of inferiority, separateness and inequality" for gays and lesbians, the San Francisco Chronicle reported Thursday.
A U.S. Supreme Court finding that same-sex marriage is a constitutional right would overturn such bans in 41 states, including California.
Protect Marriage, a conservative religious coalition defending the proposition, argued in a brief to the court that Proposition 8 did not legalize prejudice against gays and lesbians, but affirmed "society's vital interests in the uniquely procreative nature of opposite-sex relationships."
Lawyers for the couples and San Francisco, which opposes the law, have questioned the Protect Marriage's legal standing to appeal two lower court rulings striking down Proposition 8, which the state of California declined to appeal.
Protect Marriage says it has a right to represent the interests of the state and voters who support the law.