SAN FRANCISCO - The biggest social issue in this presidential race is the debate over whether gays should be allowed to marry. Louisiana voters recently tilted 4-to-1 in favor of adding a ban on same-sex marriage to their state constitution, and 71 percent of Missouri voters did the same last month. Voters in at least nine more states will weigh the question on their ballots in November, and legislation on the question has been introduced in at least 25 states this year. President Bush supports amending the U.S. Constitution to ban gay marriage. Sen. John Kerry, D-Mass., the Democratic presidential...