OLYMPIA — Microsoft Corp. CEO Steve Ballmer and co-founder Bill Gates have each donated $100,000 to the campaign supporting the state’s new gay marriage law, which faces a referendum vote in November.Zach Silk, campaign manager for Washington United for Marriage, said Monday that the checks were cut Friday and were reported to the state Public Disclosure Commission on Monday afternoon. “It’s going to make a tremendous difference,” Silk said. “It’s very important for us to have that broad support from business leaders and companies themselves.”Also Friday, Microsoft general counsel Brad Smith made a $25,000 donation to the campaign. Microsoft is just one of several prominent Pacific Northwest businesses that have expressed their support for same-sex marriage, including Amazon, Starbucks Corp. and Nike Inc. Referendum 74 was certified for the ballot last month after gay marriage opponents turned in more than 240,000 signatures, far more than the minimum of 120,577 valid voter signatures required.