WASHINGTON (Reuters) - The Supreme Court ruled on Thursday that gay marriage opponents do not have an automatic legal right to prevent public release of the names and addresses of the signers of a Washington state ballot measure favoring traditional marriage.

The high court’s 8-1 ruling was a defeat for a group called Protect Marriage Washington. It had argued that public disclosure could lead to threats, harassment or reprisal.

More than 138,000 individuals signed a petition in 2009 to put a referendum on the state ballot that would have overturned a new state law extending the benefits of marriage to couples registered as domestic partners. The referendum failed.

Some supporters of gay rights had vowed to put the names of those who signed the petition on the Internet.

Lawyers for the group argued that the signers’ constitutional right to political free-speech under the First Amendment trumped the state’s public records law.

A federal judge initially agreed with the group that disclosure of the names and addresses violated constitutional protections of anonymous political speech.

But a U.S. appeals court ruled the names could be made public and said the signatures had been gathered in public with no promise of confidentiality.

In the majority opinion written by Chief Justice John Roberts, the Supreme Court ruled that disclosure of referendum petitions generally does not violate any free-speech rights.

Only Justice Clarence Thomas dissented.

The Supreme Court case is Doe v. Reed, 09-559.