Because equal means equal, not just for those you like.
From the Washington Post:
Prop 8 overturned - Judge overturns Calif. gay marriage ban
A federal judge in California ruled Wednesday that the state's Proposition 8 ban on same-sex marriage violates the constitutional right to equal protection, the first step in a legal struggle that most expect will end at the Supreme Court.
"Proposition 8 fails to advance any rational basis in singling out gay men and lesbians for denial of a marriage license," wrote Judge Vaughn R. Walker, chief judge of the U.S. District Court for the Northern District of California in San Francisco. "Indeed, the evidence shows Proposition 8 does nothing more than enshrine in the California constitution the notion that opposite sex couples are superior to same sex couples."