"This was the crowning touch on what has been an awe-inspiring 12 months," he said.
Milk said the victories are evidence of his uncle's philosophy on gay rights.
"The successes have come from people being authentic, people being out," he said. "My uncle said 35 years ago: If they know us, they can't hate us."
Milk, who is gay himself, was 17 when his uncle, a San Francisco supervisor, was assassinated at City Hall.
He is the co-founder of the Harvey Milk Foundation and accepted a posthumous medal from President Obama in his uncle's honor. He spoke over the phone from Florida, where he lives.
He said of the Proposition 8 decision: "It is a spark of hope that can reach all of the states that do not have equality. It's a spark that can reach across the ocean."
Asked about the new legal challenge, he didn't seem surprised.
"People who divide and do not celebrate diversity are not going to give up," he said. "It doesn't end."
But events like high-profile marriages humanize gay couples and increase acceptance, he said.
"This is a tremendous amount of energy we can spread, not just to other states but around the globe," Milk said.