There are two main effects of the different treatment under federal law.
One is the tax rate. Take two couples where one partner has a taxable income of $20,000 and the other makes $40,000. If they can file their federal taxes jointly, the tax bill would be $8,217.50. Filing separately, the combined bill would be $9,032.50 -- more than $800 higher.
Another disparity comes with the federal government's treatment of employer-provided health insurance, which also affects unmarried heterosexual couples.
For example, Dan Jessup is a project manager at JPMorgan Chase in Indiana. His partner, Bob Chenoweth, is self-employed, running two businesses out of the couple's Mooresville, Indiana, home. So Chenoweth gets health insurance through Chase.
But Jessup is required to count the company's cost of his partner's benefits as additional income for tax purposes.
State and federal taxes on those benefits cost about $1,800 per year, Jessup said.
Let's see Congress fix that marriage penalty!