When William Bolthouse, a California philanthropist, donated $100,000 in March to support a proposition to ban gay marriage in California, calls and emails poured in -- not to Mr. Bolthouse, but to the corporate offices of a company that bears his name -- even though he sold it three years earlier. "It wasn't us, it's not our fault," says Jeffrey Dunn, now the chief executive of Bolthouse Farms, whose juice bottles are sold at upscale markets such as Whole Foods. Bolthouse Farms is the latest target in what has become an increasingly bitter political fight in California. As gay-rights activists...