Alan Abramowitz finds that Bush did not increase his vote anymore in states with gay marriage referenda than in the nation as a whole. Michael McDonald finds, however, that such referenda did stimulate greater turnout:
"Gay marriage was clearly pulling people to the polls," McDonald said, citing evidence that battleground states with a gay marriage saw a 6.6 percent increase in voter turnout. Voter turnout in non-battleground states with a similar initiative saw the voter turnout increase by 4.5. (By way of comparison, turnout in the battleground states generally increased by 6.9 percent, while it rose only 1.6 percent in non-battleground states without a gay marriage initiative.)
This probably helps explain why Bush increased his pluralities in Georgia by 243,000, Oklahoma by 186,000, in Kentucky by 121,000, and in Utah by 66,000. He increased his plurality in Alabama by 232,000, probably thanks in part to a statewide referendum to eliminate (moot) segregationist language from the state constitution. (It failed). These increases in uncompetitive states -- plus Bush's somewhat better performances in New York and New Jersey -- contributed substantially to his victory in the popular vote.