Gay activists often try to point to the Williams Institute's claims that redefining marriage will stimulate state economies. We recently pointed out a small business owner in Washington, DC showing how these claims turned out to be false.
Last week the Associated Press noted that California's nonpartisan Legislative Analyst's Office and Department of Finance found that "the potential financial effect [re-legalizing same-sex marriage in California] on state and local governments would be negligible over the long run." You can see a PDF of their report here.
Interestingly enough, they said the same thing about passing Prop 8 back in 2008.
Bottom line: over the long run, legalizing same-sex marriage won't create an economic boom for states. And there's plenty of evidence that it will cause real harm. That much at least is clear.