Outside of whether or not it should or shouldn’t be legal, or why big G is even in the business of issuing marriage licenses like it does for driving, how is it costing revenue? If anything, gay couples filing jointly will pay less in taxes. There is a myriad of things that couples do that divide the cost of living, hence paying less taxes. The only thing I can think of is couples will pay at the county clerk for their marriage license, but that is only one time and doesn’t benefit the federal government. Sorry, just had do show some critical thinking here.