Researchers do not know why voting results for same-sex marriage initiatives fail to match polls, Egan said, but DiCamillo said one reason for the discrepancy in 2008 was that Prop. 8 proponents launched a successful campaign that boosted voter turnout in their favor.

Marc Solomon, the marriage director of Equality California, which works to secure legal protections for gay people, said that the poll results were "great news" because they showed an increase from last year's Field Poll when 49 percent of voters approved of same-sex marriage. A Public Policy Institute of California survey released in March also showed a growing acceptance of same-sex marriage, with 50 percent of participants supporting same-sex marriage and 45 percent opposing it.

Still, Solomon acknowledged that the Field Poll showed a narrow margin and that Equality California needs to continue its outreach to voters before 2012, when supporters hope to get another initiative on the ballot.