O'Malley is expected to sign the legislation, which he sponsored, Thursday, though same-sex ceremonies cannot be performed until January 2013.
Gay marriage supporters assume they'll have another hurdle to jump as the law is expected to be petitioned to referendum in November.
Gay marriage opponents are prepared to collect the nearly 56,000 signatures needed to put the measure on the ballot and are expected to rely heavily on churchgoers who oppose same-sex marriage as a matter of faith, to reach that goal.
Organizers say they hope to collect nearly double the required number of signatures to ensure enough are validated by the state elections board.
