With more than half of voters supporting a repeal of Michigan’s gay marriage ban, advocates say it’s not a matter of if — but when — same-sex marriage is legal in the state. How soon? Gay rights activists plan a 2016 ballot drive to overturn the 2004 constitutional ban approved by voters. Democratic senators last week introduced legislation to put the gay marriage question to voters in 2014, but odds of it passing a Republican-controlled Legislature are slim. “We want to go to the ballot, win and make it a sustainable win that is an indication of a climate change in Michigan,” said Emily Dievendorf, managing director of Equality Michigan, a statewide gay rights organization.